An annular sealing device is configured to be mounted in an annular groove provided on the outer periphery of a shaft so as to seal an annular space between the shaft and a housing that rotate with respect to each other, and change the fluid pressure in the space, and maintains the fluid pressure in a target region to be sealed, and slidably contacts a sidewall surface on the low pressure side of the annular groove. The sealing device includes a sliding portion that slides on the sidewall surface; an inner periphery portion that contacts the shaft; a sliding portion groove formed in the sliding portion so as to extend in the circumferential direction, the sliding portion groove being recessed in the direction of the axis; an introduction groove formed so as to extend from the sliding portion groove to the inner periphery portion, the introduction groove being formed so as to extend from the sliding portion groove to the inner periphery portion, the introduction groove being recessed in the direction of the axis; and an inner periphery groove provided in a joined portion of the sliding portion and the inner periphery portion, the inner periphery groove being recessed in the direction of the axis.