A switching valve assembly for an endoscope (10) has a cylinder (50) and a piston unit (51) , which includes a valve head (51b) , a flow channel (71, 72), an end rod (51a) and a button device (43, 47). The piston unit is settable in an initial position, a down position and a halfway position to change over flow lines (28, 46). A biasing unit (52) of a two-stage type is secured to the cylinder on an upper side of the cylinder passage, biases the piston unit, and stops the piston unit in the halfway position when the button device is pushed in a downward direction from the initial position. A first regulating device (67, 102) prevents the piston unit from rotating relative to the biasing unit. A second regulating device (64, 94) prevents the biasing unit from rotating relative to the cylinder, so as to ensure alignment of a channel opening of the flow channel with one of the flow openings upon sliding the piston unit.
