A flow passage switching unit includes a valve body in which plural ports are formed, a spool arranged slidably in axial directions in the interior of the valve body, a spool drive unit for driving the spool in an A direction, a biasing mechanism that is configured to bias the spool elastically in the interior of the valve body, and a piston section that exerts a force on the spool in a B direction based on the pressure of a first output port.