a metering pump device capable of performing a fixed amount of discharge with a high accuracy even if a pressure difference occurs between outflow side valves . in the metering pump device , two reciprocating pump devices are used . the end and start of the discharge period of one reciprocating pump device are overlapped with the start and end of the discharge period of the other reciprocating pump device . after the inhalation operation and before the discharge period , both the inflow side valves and the outflow side valves are closed to perform a correction operation for eliminating the pressure difference by increasing or decreasing the internal volumes of the pump chambers .