A rotary valve (1) includes a valve body (2) formed with a chamber communication passage (24) for flowing a main fluid and a vent communication passage (25) for discharging the main fluid and a cylindrical valve member (3) rotatably held in the valve body (2) and formed with a main passage (31) for flowing the main fluid. The cylindrical valve member (3) is rotated to switch the connection of the main passage (31) between the chamber communication passage (24) and the vent communication passage (25). The rotary switching valve (1) further includes a purge gas passage for passing purge gas in a clearance (11) between the valve body (2) and the cylindrical valve member (3), the purge gas being supplied to the clearance (11) to prevent the main fluid from leaking out of the main passage (31).