A mechanical seal which can provide stable sealing performance over a long period of time. A rotating ring unit 100 includes an annular elastic seal 130 having a first annular part 131 to which a rotating ring 110 is fitted and thus fixed on an inner circumferential surface side thereof, a second annular part 133 provided in close contact with an outer circumferential surface of a rotating shaft, and a curved part 132 which connects the first and second annular parts 131 and 133 and follows a positional change of the second annular part 133 with respect to the first annular part 131, an annular tightening member 140 which tightens an outer circumferential surface of the second annular part 133 on the rotating shaft, and a metal case 120 including an annular fitting part 121 to which the first annular part 131 is fitted and thus fixed on the inner circumferential surface side thereof.