A material gas concentration control system for keeping a concentration of a material gas in a mixed gas constant comprising a tank to accommodate the material, an inlet line to input a carrier gas for evaporating the accommodated material into the tank, and an outlet line to output the mixed gas consisting of the material gas evaporated in the tank and the carrier gas, and further comprising a first valve arranged in the inlet line, a concentration measuring part that measures the concentration of the material gas in the mixed gas, and a concentration control part that controls an open degree of the first valve so as to make the measured concentration of the material gas measured by the concentration measuring part become the previously determined set concentration.