To provide a valve device which reduces a sliding resistance of a valve and a wear of a seat member.

The on-off valve 36b includes: a body 40 having therein a fluid passage 41 which a fluid flows through; a shaft 48 rotatably provided in the body 40; a spherical valve element 50 attached to the shaft 48 and opening and closing the fluid passage 41 according to the rotating operation of the shaft 48; and a seat mechanism 80 provided in the fluid passage 41, and having a movable side seat member 86 holding the valve element 50 by contacting the valve element 50. The seat mechanism 80 is constituted to be switchable between the contact condition in which the movable side seat member 86 is in contact with the valve element 50, and the non-contact condition in which the movable side seat member 86 is separated from the valve element 50.