diff --git a/src/genesis/parsers/sheet_nature.py b/src/genesis/parsers/sheet_nature.py
new file mode 100644
index 0000000..d77c825
--- /dev/null
+++ b/src/genesis/parsers/sheet_nature.py
@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+from enum import Enum
+from typing import Any
+
+
+class SheetNature(Enum):
+    TABLE = "table"
+    FREE_TEXT = "free_text"
+    MIXED = "mixed"
+
+
+def _non_empty(row: list[Any]) -> list[Any]:
+    return [c for c in row if c is not None and str(c).strip() != ""]
+
+
+def find_header_row(matrix: list[list[Any]]) -> int:
+    for i, row in enumerate(matrix):
+        if len(_non_empty(row)) >= 2:
+            return i
+    return -1
+
+
+def _free_text_like(matrix: list[list[Any]]) -> bool:
+    if not matrix:
+        return True
+    max_cols = max((len(row) for row in matrix), default=0)
+    if max_cols <= 1:
+        return True
+    non_empty_rows = [r for r in matrix if _non_empty(r)]
+    if len(non_empty_rows) / len(matrix) < 0.7:
+        return True
+    return find_header_row(matrix) == -1
+
+
+def classify_sheet(matrix: list[list[Any]]) -> SheetNature:
+    if _free_text_like(matrix):
+        return SheetNature.FREE_TEXT
+    header_row = find_header_row(matrix)
+    if header_row >= 0:
+        for row in matrix[header_row + 1:]:
+            if any(str(c).strip().startswith(("・", "■")) for c in _non_empty(row)):
+                return SheetNature.MIXED
+    return SheetNature.TABLE
\ No newline at end of file
diff --git a/tests/test_sheet_nature.py b/tests/test_sheet_nature.py
new file mode 100644
index 0000000..8320271
--- /dev/null
+++ b/tests/test_sheet_nature.py
@@ -0,0 +1,22 @@
+from genesis.parsers.sheet_nature import SheetNature, classify_sheet, find_header_row
+
+
+def test_table_detection():
+    m = [["機能ID", "機能名"], ["A001", "社員登録"], ["A002", "退職処理"]]
+    assert classify_sheet(m) == SheetNature.TABLE
+
+
+def test_free_text_single_col():
+    m = [["新入社員を登録できる。"], ["氏名・所属・入社日を入力する。"]]
+    assert classify_sheet(m) == SheetNature.FREE_TEXT
+
+
+def test_free_text_many_empty_rows():
+    m = [["要求A"], [], [], [], ["要求B"]]
+    assert classify_sheet(m) == SheetNature.FREE_TEXT
+
+
+def test_header_row_index():
+    m = [["機能ID", "名前"], ["1", "田中"]]
+    assert find_header_row(m) == 0
+    assert find_header_row([["自由テキスト"]]) == -1
\ No newline at end of file