A valve device includes: a ball valve having a ball face shaped in a convex spherical surface; and a valve seat having a seat face shaped in a concave spherical surface. The seat face is pressed onto the ball face. The ball valve is rotated to open a valve by communicating a valve opening defined in the ball valve and a seat opening defined in the valve seat with each other, and to close the valve by stop the communication. A diameter of the seat opening ϕ2 is smaller than a diameter of the valve opening ϕ1. A curvature radius of the ball face R1 is smaller than or equal to a curvature radius of the seat face is R2.