To provide a pump gun, convenient, efficient and portable, saving the output of the physical strength of a user and ensuring the sufficient safety of the user in the operating process of a pump.

A driving board is fastened to a main shaft, a steel ball clutch is connected to one face of a driven gear 34, another face of the driven gear is linked to a pressure control spring and a rotary handle is operated for controlling back pressure applied to the steel ball clutch by the pressure control spring. A crank is formed at the terminal end of the main shaft, the crank is joined to a connecting rod 6 via a crank pin and the connecting rod is connected to a piston 7 arranged in a fixed cylinder via a connecting rod pin. The main shaft is driven via the driving board, the piston is reciprocated in the cylinder 8 and air is compressed and forced out of a hole in the front end of the cylinder to complete pumping operation. When pressure at the forced end once exceeds a set value, stress formed thereby makes the steel ball clutch jump out and the driven gear at the terminal end of a gear system run idle, thereby preventing excessive pumping operation.