A valve device includes: a valve having a ball surface having a convex spherical shape, and a first opening opened in the ball surface to allow fluid to pass through; and a seat having an annular shape and opposing the ball surface to be in sliding contact, the seat being pressed against the ball surface, the seat having a second opening to communicate with the first opening. The seat has a sealing surface in a sliding contact with the ball surface at a valve closed time when the ball surface and the second opening overlap with each other, and a seat contact surface in a sliding contact with an opening inner wall surface or an opening end surface of the first opening at a valve opened time when the first opening and the second opening overlap with each other.