diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..0686f61
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,125 @@
+use indextree::NodeId;
+
+use crate::elements::*;
+use crate::Org;
+
+/// Orgize Error
+#[derive(Debug)]
+pub enum OrgizeError {
+    /// Expect this node has children
+    Children { at: NodeId },
+    /// Expect this node has no children
+    NoChildren { at: NodeId },
+    /// Expect this node contains a headline or section element
+    HeadlineOrSection { at: NodeId },
+    /// Expect this node contains a title element
+    Title { at: NodeId },
+    /// Expect this node contains a headline element
+    Headline { at: NodeId },
+    /// Expect a detached headline
+    Detached { at: NodeId },
+    /// Expect a headline where its level >= max and <= min
+    HeadlineLevel {
+        max: Option<usize>,
+        min: Option<usize>,
+        at: NodeId,
+    },
+}
+
+impl Org<'_> {
+    pub fn check(&self) -> Result<(), OrgizeError> {
+        for node_id in self.root.descendants(&self.arena) {
+            let node = &self.arena[node_id];
+            match node.get() {
+                Element::Document => {
+                    for child_id in node_id.children(&self.arena) {
+                        match self.arena[child_id].get() {
+                            Element::Headline { .. } | Element::Section => (),
+                            _ => return Err(OrgizeError::HeadlineOrSection { at: child_id }),
+                        }
+                    }
+                }
+                Element::Headline { .. } => {
+                    if node.first_child().is_none() {
+                        return Err(OrgizeError::Children { at: node_id });
+                    }
+                    let title = node.first_child().unwrap();
+                    match self.arena[title].get() {
+                        Element::Title(Title { .. }) => (),
+                        _ => return Err(OrgizeError::Title { at: title }),
+                    }
+                    if let Some(next) = self.arena[title].next_sibling() {
+                        match self.arena[next].get() {
+                            Element::Headline { .. } | Element::Section => (),
+                            _ => return Err(OrgizeError::HeadlineOrSection { at: next }),
+                        }
+
+                        for sibling in next.following_siblings(&self.arena).skip(1) {
+                            match self.arena[sibling].get() {
+                                Element::Headline { .. } => (),
+                                _ => return Err(OrgizeError::Headline { at: sibling }),
+                            }
+                        }
+                    }
+                }
+                Element::Title(Title { raw, .. }) => {
+                    if !raw.is_empty() && node.first_child().is_none() {
+                        return Err(OrgizeError::Children { at: node_id });
+                    }
+                }
+                Element::CommentBlock(_)
+                | Element::ExampleBlock(_)
+                | Element::ExportBlock(_)
+                | Element::SourceBlock(_)
+                | Element::BabelCall(_)
+                | Element::InlineSrc(_)
+                | Element::Code { .. }
+                | Element::FnRef(_)
+                | Element::InlineCall(_)
+                | Element::Link(_)
+                | Element::Macros(_)
+                | Element::RadioTarget
+                | Element::Snippet(_)
+                | Element::Target(_)
+                | Element::Text { .. }
+                | Element::Timestamp(_)
+                | Element::Verbatim { .. }
+                | Element::FnDef(_)
+                | Element::Clock(_)
+                | Element::Comment { .. }
+                | Element::FixedWidth { .. }
+                | Element::Keyword(_)
+                | Element::Drawer(_)
+                | Element::Rule
+                | Element::Cookie(_)
+                | Element::Table(Table::TableEl { .. })
+                | Element::TableRow(TableRow::Rule) => {
+                    if node.first_child().is_some() {
+                        return Err(OrgizeError::NoChildren { at: node_id });
+                    }
+                }
+                Element::SpecialBlock(_)
+                | Element::QuoteBlock(_)
+                | Element::CenterBlock(_)
+                | Element::VerseBlock(_)
+                | Element::Paragraph
+                | Element::Section
+                | Element::Table(Table::Org { .. })
+                | Element::TableRow(TableRow::Standard)
+                | Element::TableCell
+                | Element::Bold
+                | Element::Italic
+                | Element::Underline
+                | Element::Strike
+                | Element::DynBlock(_)
+                | Element::List(_)
+                | Element::ListItem(_) => {
+                    if node.first_child().is_none() {
+                        return Err(OrgizeError::Children { at: node_id });
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
+}