In an embodiment, a mechanical seal includes: a bellows 10 sealing between a rotating-side seal ring 5 biased in the axial direction, and an outer periphery face of a rotational shaft 2; a case 11 fitted to the ring 5 and the outer periphery of the bellows 10; and a drive ring 12 tightening the bellows 10 to the shaft 2, wherein cutout grooves 5b running through in the axial direction are provided on the outer periphery area of the ring 5; detent cutout areas 5d are provided on the side face of the ring 5 on a sealing face S side in a manner extending in the circumferential direction from the cutout grooves 5b; and detent convex areas 11e are provided at the edge area of the case 11 on the ring 5 side in a manner facing the cutout grooves 5b and bulging in the inner-diameter direction.