The present invention relates to a dosing mechanism including: a body; and a pump unit (3) that is attached to the body, in which the pump unit (3) includes: a pump (31) in which a volume of a solution feeding part therein repeats expansion and shrinkage, thereby sucking a medicine from a medicine vessel and discharging the medicine to a patient; a suction-side pipe (333) that extends from the pump (31) toward the medicine vessel; a discharge-side pipe (334) that extends from the pump (31) toward the patient; a relief pipe (35, 35a) whose one end (351) is connected to the discharge-side pipe (333) while the other end (352) is located in a part whose pressure is lower than pressure inside the discharge-side pipe when the medicine is dosed; and a relief pipe on-off valve (36, 36a) that opens and closes the relief pipe (35, 35a), a use method of a dosing mechanism, and a pump unit for a dosing mechanism.
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