To provide a metering pump having durability and profitability capable of use in universal applications by developing a piston type pump without need for packing, while taking advantage of characteristics of a plunger pump having a simple and high precision structure easily manufactured, since piston pumps and plunger pumps need the packing in the structure, but existence of the packing raises the problems of maintenance and durability of the pump as well as becomes inevitable weak point of being the cause of leakage and pollution of liquid feed.

A retractable cylindrical diaphragm is coated and fixed on the front end of the plunger, and the opening portion of the diaphragm is fixed on the opening portion of the cylinder. Thereby, a diaphragm plunger pump mechanism having an excellent metering pump mechanism is provided, in which the plunger is indirectly connected to the liquid feed through the diaphragm in the cylinder, and the plunger pump without need for a rod packing is provided.