Disposable mechanism for taking out a fixed amount of fluid and system for supplying a fixed amount of fluid

There is provided a system for supplying a fixed amount of fluid in which the reverse flow of the fluid is eliminated and a precise fixed amount of the fluid can be delivered. In a system for supplying a fixed amount of fluid comprising a fluid accommodating unit comprising a fluid accommodating bag and a pump connected to the fluid accommodating bag and a fluid supplying device body in which the fluid accommodating unit is detachably disposed and a pump driving mechanism for driving the pump is disposed, the pump of the fluid accommodating unit is a bellows pump having two check valves; the check valve comprises a valve base having through-holes formed therein and whose downstream surface is curved to be convex toward the downstream and a valve body made from an elastic thin plate for covering the downstream surface of the valve base and having a hole at the center thereof; and the pump driving mechanism drives a bellows so as to expand and contract it at a predetermined stroke.