A rotary valve comprising sliding members for supplying treatment gas containing water vapor to a plurality of treatment processes is provided. The rotary valve comprises sliding members (1, 2; 4, 6), through which open flow passages (8, 9, 10), having sliding surfaces slidable to each other and a mechanical seal mechanism for switching the flow passages (8, 9, 10) while sealing fluid by preliminarily pressing the sliding surfaces, wherein the sliding surface of each sliding members (1, 2; 4, 6) is made of hydrophobic material (2, 4) different in hardness from the other.