To improve cleaning effects by generating a flow of cleaning water also on the upper surface of a piston 2.

A piston-type quantitative filling device includes: a cylinder 6 communicating with a filled liquid tank 4; a filling nozzle 8 communicating with the cylinder 6; a flow path switching valve 14 (first open/close valve 16 and second open/close valve 18) for alternately connecting and shutting between the tank 4 and the cylinder 6 and between the cylinder 6 and the nozzle 8; and a safety valve 22 for discharging a filled liquid when the inside of the cylinder 6 abnormally rises in pressure. The safety valve 22 has an air passage 38 connected in addition to a conventional liquid discharging passage 36, so that the discharging passage 36 and the air passage 38 can be switched for connection by a three-way valve 40. In cleaning, the safety valve 22 is kept open by a stopper 48, and the piston 2 is raised/lowered while feeding the air into a cylinder chamber 20 so as to suck and discharge the cleaning water.