To provide a gasket which prevents staggered inclination of both wall surface sides of a mounting groove and causes no compression under a twisted state even when a chamfering surface is formed at a corner of an opening in the mounting groove.

The gasket 3 includes left protrusions 33a, 33b and a right protrusion 34. The left protrusions 33a, 33b are formed at a predetermined interval in a depth direction of a mounting groove 11 and abut on a left side wall surface 110 in a state before being compressed. The right protrusion 34 abuts on a right side wall surface 111 in a state before being compressed. Two left protrusions are formed in the depth direction of the mounting groove 11 to abut on the left side wall surface 110 of the mounting groove 11. Thus even though chamfering surfaces 115, 116 are formed, the inclination of the mounting groove 11 to the left side wall surface 110 can be suppressed, thereby preventing staggered inclination of the gasket 3 to the left side wall surface 110 side and right side wall surface 111 side of the mounting groove and cause no compression under a twisted state.