The on-off valve includes a body having a fluid path, a spherical valve body opening and closing the fluid path, a guide member opposing the valve body and having a flange portion radially extending outward, a first seat member slidable along an outer peripheral surface of the guide member and seating the valve body, an O-ring disposed between the first seat member and the guide member, a chamber formed of an inner wall portion of the fluid path, the outer peripheral surface, the flange portion, and the first seat member, and a spring member disposed in the chamber and urging the first seat member toward the valve body. In the on-off valve, a gap portion is formed between the inner wall portion and the first seat member and connects the valve body side of the fluid path to the chamber.