A switching valve assembly for an endoscope (10) includes a cylinder (50), a piston unit (51), a button device (43, 47), a flow channel (71, 72) in the piston unit, and plural flow openings (56, 57). The piston unit changes over plural flow lines (28, 46) between communication and interruption by changing a combination of the flow openings with the flow channel for communication. The switching valve assembly includes a cylinder cap device (76) and a slide device (77) of a cup shape. A ring projection (107a) of the button device is directed downwards. A cup plate portion (77b) of the slide device contacts the ring projection when the piston unit is slid down to a halfway position. Seal packing (109) deforms resiliently to seal the ring projection and cup plate portion together in an air-tight manner. A vent hole (103) is formed through the cup plate portion, surrounded by the seal packing, for connection with one of the plural flow lines to come open to atmosphere.
