PROBLEM TO BE SOLVED: To provide a suction button of an endoscope, which controls suction of a body fluid etc. from a suction port opened at a leading end of an endoscope insertion portion, and which enlarges the diameter of an inner pipeline in a plunger.SOLUTION: A valve guide member 30 of a cylinder unit 24 includes a first large-diameter pipeline 40 communicating with a suction channel 17, a second small-diameter pipeline 41, and a third cone-shaped pipeline 42 for connecting both the pipelines 40 and 41 together. The inner wall of the third pipeline 42 is provided with a connection port 45 communicating with a negative pressure source channel 21. The valve guide member 30 is provided with the plunger 25 which has an inner pipeline 60 and which opens/closes the connection port 45. The valve guide member 30 is provided with a ventilation channel 78 for connecting an opening 76 for ventilation, which is opened in a position to face a cap 26, and an in-channel opening 77 which is opened in the negative pressure source channel 21. The cap 26 is elastically deformed when the plunger 25 is pushed, so that the opening 76 for ventilation can be closed with an inclined surface 74 thereof. When the pushing operation is released, the cap 26 is restored to an original shape to open the opening 76 for ventilation. The ventilation channel does not need to be formed in the plunger 25.
