To provide a washer which allows a gap generated between a fixture and the washer to be filled with a packing material so as to improve water stop function, and which has excellent strength against compression.

A washer body is formed by a dome-shaped wall surface 5. A through hole 7 is disposed at a top portion of the wall surface 5. Recessed portions 8 are respectively disposed at a plurality of positions on an external surface of the wall surface 5. The recessed portions 8 have a spiral shape in a plan view that is long along a circumferential direction of the wall surface 5, and protrude beyond an internal surface of the wall surface 5.