To provide a pump unit that is capable of feeding water stably as there are only minute pressure fluctuations produced by increasing or decreasing the operating frequency in a fixed quantity, even if customer water is continuously used in a small quantity by judging the shutdown control of a pump unit through the operating frequency and the detection pressure value of a pressure sensor, and further to disuse any flow switch for the costdown of a product.

After operation starting, a pump unit becomes less than the frequency (stop judgment enabling frequency fe) where operating frequency (f) is set up and that at a point of time when any change in the detection pressure value of the pressure sensor becomes unrecognized, the operating frequency (f) is once increased as much as Δf, making the pressure value go up, and once being on standby, at a time when the operating frequency (f) is decreased as much as Δf, pump operation is stopped at a point of time checking a fact that the pressure value is not varied any longer.