A mechanical seal comprising: a rotation-side member including a cylindrical portion fixed to a rotating shaft of fluid equipment and a sealing face , the rotation-side member 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 and being slidably contactable with the sealing face of the rotation-side member , 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 ; the cylindrical portion includes an engaged portion ; the cylindrical portion includes a recessed surface provided on the opposite side of the engaged portion from the stationary-side member in the axial direction and recessed radially inward from an outer circumferential surface of the engaged portion ; an outer diameter of the engaged portion of the cylindrical portion is greater than an inner diameter of the engaging portion of the inner circumferential portion ; the engaged portion is continuously formed on the cylindrical portion over an entire circumference thereof; the engaging portion and the engaged portion come into contact with each other to bring the rotation-side member 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 including a cylindrical portion fixed to a rotating shaft of fluid equipment and a sealing face , the rotation-side member 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 and being slidably contactable with the sealing face of the rotation-side member , 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 ; the cylindrical portion includes an engaged portion ; the cylindrical portion includes a recessed surface provided on the opposite side of the engaged portion from the stationary-side member in the axial direction and recessed radially inward from an outer circumferential surface of the engaged portion ; an outer diameter of the engaged portion of the cylindrical portion is greater than an inner diameter of the engaging portion of the inner circumferential portion ; the engaged portion is continuously formed on the cylindrical portion over an entire circumference thereof; the engaging portion and the engaged portion come into contact with each other to bring the rotation-side member 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 . <EOS>