To provide a mechanical seal in which degradation or tearing of a bellows does not occur, the spring constant of the bellows is reduced, and the fluctuation in the load due to stretching and contracting of the bellows is reduced. An inside-type mechanical seal comprising a sleeve (2, 20) fixed to a rotating shaft, and a cartridge (4, 40) fixed to a housing (3); a rotation-side sealing ring (5) being provided to the sleeve (2, 20); and a fixed-side sealing ring (6), which slides against the rotation-side sealing ring (5), and a bellows (7, 15, 30) for axially urging the fixed-side sealing ring (6), being provided to the cartridge (4, 40); the mechanical seal being characterized in that: the bellows (7, 15, 30) is made from a metal; and the metal bellows (7, 15, 30) has one end fixed to a case (8) for holding the fixed-side sealing ring (6) and another end fixed to the cartridge (4, 40), the plate thickness of the metal bellows (7, 15, 30) being set so as to be smaller than the plate thickness of the case (8) or the cartridge (4, 40).