The present invention provides a mechanical seal preventing the deformation of the cartilage and exhibiting a good sealing performance. In said mechanical seal, the deformation of the back portion of the cartilage due to the force from the pump housing is absorbed by forming a groove portion having cross section substantially a triangular shape at the back portion of the cartilage. As a result, the deformation hardly transfers to the area near the inner tube portion of the cartilage 16 and the inner tube portion of the back portion; and the relative position of the seal ring, and the bellows or so scarcely changes with respect to the structure of the mating ring or so on the side of the rotation axis, thus the assembling length and the assembled load of the mechanical seal does not change, which allows the mechanical seal to exhibit good sealing performance.