A mechanical seal (1, 11) comprising: a rotation-side member (2, 12, 22) including a cylindrical portion (2b, 12b, 22b) fixed to a rotating shaft of fluid equipment and a sealing face (2s, 12s), the rotation-side member (2, 12, 22) being rotatable with the rotating shaft; and a stationary-side member including a seal case fixed to a housing of the fluid equipment and having elasticity, a stationary ring having a sealing face (2s, 12s, 22s) and being slidably contactable with the sealing face of the rotation-side member (2, 12, 22), and a biasing means held in the seal case to bias the stationary ring so that the stationary ring is movable in an axial direction of the rotating shaft, the seal case includes an inner circumferential portion extending along the rotating shaft; the inner circumferential portion includes an engaging portion (6d, 6dw); the cylindrical portion (2b, 12b, 22b) includes an engaged portion (2q, 12q, 22q, 2r, 22r, 12t); the cylindrical portion (2b, 12b, 22b) includes a recessed surface provided on the opposite side of the engaged portion (2q, 12q, 22q, 2r, 22r, 12t) from the stationary-side member in the axial direction and recessed radially inward from an outer circumferential surface of the engaged portion (2q, 12q, 22q, 2r, 22r, 12t); an outer diameter of the engaged portion (2q, 12q, 22q, 2r, 22r, 12t) of the cylindrical portion (2b, 12b, 22b) is greater than an inner diameter of the engaging portion (6d, 6dw) of the inner circumferential portion; the engaged portion (2q, 12q, 22q, 2r, 22r, 12t) is continuously formed on the cylindrical portion (2b, 12b, 22b) over an entire circumference thereof; the engaging portion (6d, 6dw) and the engaged portion (2q, 12q, 22q, 2r, 22r, 12t) come into contact with each other to bring the rotation-side member (2, 12, 22) and the stationary-side member into engagement with each other; characterized in that an O-ring is interposed as a second seal between an inner circumferential portion of the seal case and the stationary ring. a rotation-side member (2, 12, 22) including a cylindrical portion (2b, 12b, 22b) fixed to a rotating shaft of fluid equipment and a sealing face (2s, 12s), the rotation-side member (2, 12, 22) being rotatable with the rotating shaft; and a stationary-side member including a seal case fixed to a housing of the fluid equipment and having elasticity, a stationary ring having a sealing face (2s, 12s, 22s) and being slidably contactable with the sealing face of the rotation-side member (2, 12, 22), and a biasing means held in the seal case to bias the stationary ring so that the stationary ring is movable in an axial direction of the rotating shaft, the seal case includes an inner circumferential portion extending along the rotating shaft; the inner circumferential portion includes an engaging portion (6d, 6dw); the cylindrical portion (2b, 12b, 22b) includes an engaged portion (2q, 12q, 22q, 2r, 22r, 12t); the cylindrical portion (2b, 12b, 22b) includes a recessed surface provided on the opposite side of the engaged portion (2q, 12q, 22q, 2r, 22r, 12t) from the stationary-side member in the axial direction and recessed radially inward from an outer circumferential surface of the engaged portion (2q, 12q, 22q, 2r, 22r, 12t); an outer diameter of the engaged portion (2q, 12q, 22q, 2r, 22r, 12t) of the cylindrical portion (2b, 12b, 22b) is greater than an inner diameter of the engaging portion (6d, 6dw) of the inner circumferential portion; the engaged portion (2q, 12q, 22q, 2r, 22r, 12t) is continuously formed on the cylindrical portion (2b, 12b, 22b) over an entire circumference thereof; the engaging portion (6d, 6dw) and the engaged portion (2q, 12q, 22q, 2r, 22r, 12t) come into contact with each other to bring the rotation-side member (2, 12, 22) and the stationary-side member into engagement with each other; characterized in that an O-ring is interposed as a second seal between an inner circumferential portion of the seal case and the stationary ring.