A rotary valve having sliding members for supplying a water vapor-containing processinggas to processing steps. The rotary valve has a mechanical seal mechanism havingthe sliding members (1, 2 4, 6) in which flow passages (8, 9, 10) are opened andhaving sliding surfaces sliding on each other. The mechanism changes over flowpassages (8, 9, 10) by sliding while sealing fluid by applying pressure to thesliding surfaces. The sliding surfaces of the sliding members (1, 2 4, 6) areformed of hydrophobic members (2, 4) with different hardness.
