A sealing device (100) includes a resinous sealing ring (110) and an elastomeric elastic ring (120) in an annular groove 210. The sealing ring (110) has a cylindrical surface portion (112a) making intimate contact with a groove bottom of the annular groove (210) and an inclined surface portion (112b) expanding in diameter toward the atmosphere side (A). The elastic ring (120) is disposed such that at least its portion is compressed between the inclined surface portion (112b) and the groove bottom of the annular groove (210) so as to press a portion of the sealing ring (110) on the atmosphere side (A) toward the outer peripheral side.