A switching valve assembly for an endoscope, including a cylinder having a cylinder passage and a plurality of flow openings (56, 57) formed inside said cylinder passage, plural flow lines (28, 46) extending from respectively said flow openings, said switching valve assembly comprising: a piston unit contained in said cylinder passage in a slidable manner, said piston unit including a valve head, a flow channel (71, 72) formed in said valve head, an end rod, formed to extend from said valve head in an upward direction, for protruding from said cylinder passage, and a button device (43, 47) formed on a tip of said end rod; wherein said piston unit slides to an initial position, a down position lower than said initial position and where said button device 47 is positioned nearest to said cylinder passage, and a halfway position defined between said initial position and said down position, for changing a combination of said flow openings with said flow channel to change over communication and interruption between said flow lines; a biasing means of a two-stage type, secured to said cylinder on an upper side of said cylinder passage, for biasing said piston unit in said upward direction from said down position, said biasing means having smaller force of bias above said halfway position than under said halfway position, for stopping said piston unit in said halfway position when said button device is pushed from said initial position and slid in a downward direction; regulating means adapted to prevent relative rotation between the piston unit and the cylinder, the regulating means being characterised by a first regulating means (67, 102) adapted to prevent said piston unit from rotating relative to said biasing means; and a second regulating means (64, 94) adapted to preventing said biasing means from rotating relative to said cylinder, so as to ensure alignment of a channel opening of said flow channel with one of said flow openings upon sliding said piston unit. a piston unit contained in said cylinder passage in a slidable manner, said piston unit including a valve head, a flow channel (71, 72) formed in said valve head, an end rod, formed to extend from said valve head in an upward direction, for protruding from said cylinder passage, and a button device (43, 47) formed on a tip of said end rod; wherein said piston unit slides to an initial position, a down position lower than said initial position and where said button device 47 is positioned nearest to said cylinder passage, and a halfway position defined between said initial position and said down position, for changing a combination of said flow openings with said flow channel to change over communication and interruption between said flow lines; a biasing means of a two-stage type, secured to said cylinder on an upper side of said cylinder passage, for biasing said piston unit in said upward direction from said down position, said biasing means having smaller force of bias above said halfway position than under said halfway position, for stopping said piston unit in said halfway position when said button device is pushed from said initial position and slid in a downward direction; regulating means adapted to prevent relative rotation between the piston unit and the cylinder, the regulating means being characterised by a first regulating means (67, 102) adapted to prevent said piston unit from rotating relative to said biasing means; and a second regulating means (64, 94) adapted to preventing said biasing means from rotating relative to said cylinder, so as to ensure alignment of a channel opening of said flow channel with one of said flow openings upon sliding said piston unit.