A switching valve assembly for an endoscope includes a cylinder, a piston unit, a button device, a flow channel in the piston unit, and plural flow openings. The piston unit changes over plural flow lines 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 and a slide device of a cup shape. A ring projection of the button device is directed downwards. A cup plate portion of the slide device contacts the ring projection when the piston unit is slid down to a halfway position. Seal packing deforms resiliently to seal the ring projection and cup plate portion together in an air-tight manner. A vent hole 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.
