A housing unit includes a cylindrical passage member, through which cooling water flows. A valve unit includes a valve main body rotatable around its axis and a valve opened portion formed in an outer peripheral surface of the valve main body, through which the cooling water can pass. A valve seat unit includes a positioned member and a valve seat surface. An outer peripheral surface of the positioned member is positioned in a radial direction by an inner peripheral surface of the cylindrical passage member and the positioned member is movable in an axial direction in a reciprocal manner. The valve seat surface holds a fluid tight condition between the valve seat surface and the valve main body when it is in contact with the outer peripheral surface of the valve main body. A biasing member biases the valve seat unit in such a way that the valve seat surface is pushed to the outer peripheral surface of the valve main body. The cylindrical passage member includes a groove formed in an inner peripheral surface thereof at least in a part of an overlapping area, at which the cylindrical passage member and the positioned member overlap each other in the axial direction. The groove is recessed in a radial-outward direction from the inner peripheral surface.