To provide a mechanical seal which can retain a gap between sealing end surfaces in a proper non-contact state or a contact state under a condition where the temperature and/or the pressure of a fluid to be sealed is significantly fluctuated, and demonstrate an excellent and a stable sealing functions.

The fluid to be sealed is sealed at a relative rotation part of opposing end faces 3a and 4a between a first sealing ring 3 fixed on one of a seal case 2 and a rotary shaft 1 passing therethrough, and a second sealing ring 4 retained on the other side so as to be movable in the axial direction and also not to be relatively rotatable via a retention ring 5. The second sealing ring 4 and the retention ring 5 are connected with distortion in a diameter direction of both the rings 4 and 5 mutually interfered by bringing the opposing end faces 4e and 5c of both the rings 4 and 5 into contact therewith with pressure via an O ring 9. The second sealing ring 4 is made of carbon and the retention ring 5 is composed of titanium that a coefficient of thermal expansion and/or a Young's modulus is approximate to a component of the second sealing ring 4.