A spool switching valve device may include a columnar spool housing and spherical valve bodies. The spool housing includes first input port cavity portions, valve body movable portions and valve seat members. Each valve seat member has a through hole and includes a second input port portion. A second valve seat portion is provided to open and close the gap between valve body movable portion and the second input port portion. Each valve body movable portion includes an output port penetrating the side surface of the spool housing to communicate with the outside of the spool. Each valve seat mounting portion includes an engagement receiving portion suppressing the detachment of the valve seat members when the valve seat member is actually mounted. Each valve seat member includes an engagement portion fitted into the engagement receiving portion when the valve seat member is mounted to the valve seat mounting portion.