Plunger, instrument, and mold

An instrument for injecting or suctioning a fluid, the instrument comprising:

• a cylindrical member;
• and
• a plunger configured to be attached to a tip of a pin and to slide inside of the cylindrical member,
• wherein the plunger includes:
• a plunger body made of a resin and including a recess that is open at a base end, the recess being provided for inserting a part of the pin;
• a first surface disposed on an outer peripheral surface of configured to come into contact with an inner peripheral surface of the cylindrical member;
• a second surface disposed on a base end side relative to the first surface in the outer peripheral surface of the plunger body, the second surface having an outer diameter that is smaller than that of the first surface;
• an outer stepped surface disposed between the first surface and the second surface in the outer peripheral surface of the plunger body;
• a first housing disposed in the same position as the outer stepped surface in the recess in the sliding direction of the plunger, the first housing being provided for housing a tip part of the pin;
• a second housing disposed on the base end side relative to the first housing in the recess, the second housing having an inner diameter that is smaller than a largest inner diameter of the first housing;
• an inner stepped surface disposed between the first housing and the second housing in the recess, and
• an outer diameter of a part of the second surface increases when the part of the pin is inserted into the recess,
• wherein in a state where the part of the pin is inserted into the recess, at least a part of the first surface and at least a part of the second surface come into contact with the inner peripheral surface of the cylindrical member while the outer stepped surface does not come into contact with the inner peripheral surface of the cylindrical member.