To easily mix chemicals in accordance with the changed mixing ratio even when the mixing ratio changes.

This device mixes two kinds of chemicals, e.g. chemicals and pure water and is equipped with a tank 24 for mixing which stores the chemicals and pure water, a chemicals duct 29 which feeds the chemicals to the mixing tank 24, a pure water duct 30 which feeds the pure water to the mixing tank 24, a chemicals valve 31 provided to the chemicals duct 29, a pure water valve 32 provided to the pure water duct 30, a flowmeter 33 which measures the flow rate of the pure water supplied to the mixing tank 24 through the pure water valve 32, a float sensor 26 for chemicals which detects the liquid level of the chemicals and pure water in the mixing tank reaching a specific position, and a control part which controls the opening and closing of the chemicals valve 31 and pure water valve 32 according to the measurement results of the flowmeter 33 and float sensor 26 for chemicals.