To provide an inexpensive and simple structure metering pump not including a mechanical sliding part and suitable for metering highly flammable organic solvent.

This pump is provided with an air introduction part 10, a pressure reducing chamber 20 communicated to the air introduction part 10 via a suction passage 17 and reducing pressure by introducing pressurized air to the air introduction part 10, a cylinder part 30 provided with a suction port 31 and a delivery port 33, a communication pipe 40 connecting the pressure reducing chamber 20 and a cylinder part 30, and a float 25 arranged in the pressure reducing chamber 20. Pressure in the pressure reducing chamber 20 and the cylinder part 30 is reduced by introducing pressurized air to the air introduction part 10. Consequently, liquid is sucked in the cylinder part 30 from the suction port 31, sucked liquid is introduced into the pressure reducing chamber 20 through the communication pipe 40 and the float 25 floats to block the suction passage 17. Consequently, liquid of fixed quantity is sucked and retained, and an inside of the pressure reducing chamber 20 is released from a pressure reduced condition by stopping pressurized air supply, and liquid in the cylinder is delivered from the delivery port 33.