A suction button assembly of a valve structure for an endoscope includes a cylinder housing, cylinder passage, plunger or piston unit, cylinder opening, plunger stem, flow opening, valve sleeve and flow channel. A valve opening is formed in a side surface of the valve sleeve at an end of the flow channel, closed by a wall of the cylinder passage when the plunger is in the closed position, for shut-off between the suction channel and the discharge conduit, and registered with the flow opening when the plunger is in the open position, for communication between the suction channel and the discharge conduit. A valve engaging surface is formed partially with the cylinder passage, has the flow opening, and has an inner diameter decreasing in an upward direction. The valve sleeve is tapered and tightly contacted on the valve engaging surface when the plunger is in the closed position.
