A switching valve assembly for an endoscope, including a cylinder, a piston unit disposed in said cylinder in a slidable manner, a button device (43, 47) secured to an upper end of said piston unit, a flow channel (71, 72) formed in said piston unit, plural flow openings (56, 57) formed in said cylinder, wherein said piston unit is set in an initial position at an upper end of said cylinder, a down position, and a halfway position predetermined between said initial position and said down position upon operation of said button device, and changes over plural flow lines (28, 46) between communication and interruption by changing a combination of said flow openings with said flow channel for communication, said switching valve assembly comprising: a cylinder cap device, secured to said cylinder under said button device, for partially covering said piston unit; a slide device of a cup shape, contained in said cylinder cap device, having a receiving hole, for receiving entry of said piston unit, for guiding said piston unit in a longitudinal direction thereof, and for sliding in a downward direction in said cylinder cap device when said button device is pushed in said downward direction; an end ring for keeping said slide device contained in said cylinder cap device; a first coil spring, disposed between said slide device and said cylinder cap device, for biasing said slide device in an upward direction from said cylinder; a second coil spring, disposed between said button device and said slide device; a first contact surface disposed on said button device, and opposed to said slide device; a second contact surface, disposed on said slide device, for contacting said first contact surface when said piston unit is slid to said halfway position by shift of said button device; characterised in that the second coil spring is adapted for biasing said button device in said upward direction with smaller force of bias than said first coil spring and in that the switching valve assembly further comprises a seal packing, associated with one of said first and second contact surfaces, for deforming resiliently to seal said first and second contact surfaces together in an air-tight manner upon contacting one another; and vent hole, formed in said slide device through said second contact surface, surrounded by said seal packing, for connection with one of said plural flow lines to come open to atmosphere. a cylinder cap device, secured to said cylinder under said button device, for partially covering said piston unit; a slide device of a cup shape, contained in said cylinder cap device, having a receiving hole, for receiving entry of said piston unit, for guiding said piston unit in a longitudinal direction thereof, and for sliding in a downward direction in said cylinder cap device when said button device is pushed in said downward direction; an end ring for keeping said slide device contained in said cylinder cap device; a first coil spring, disposed between said slide device and said cylinder cap device, for biasing said slide device in an upward direction from said cylinder; a second coil spring, disposed between said button device and said slide device; a first contact surface disposed on said button device, and opposed to said slide device; a second contact surface, disposed on said slide device, for contacting said first contact surface when said piston unit is slid to said halfway position by shift of said button device; characterised in that the second coil spring is adapted for biasing said button device in said upward direction with smaller force of bias than said first coil spring and in that the switching valve assembly further comprises a seal packing, associated with one of said first and second contact surfaces, for deforming resiliently to seal said first and second contact surfaces together in an air-tight manner upon contacting one another; and vent hole, formed in said slide device through said second contact surface, surrounded by said seal packing, for connection with one of said plural flow lines to come open to atmosphere.