To implement high efficiency of a compressor even if a flow rate is low as well as efficiency when the flow rate is high, and reduce the size of the compressor and elongate service life of the compressor.

A compressor unit comprises: the compressor; a motor; a pressure detection means detecting discharge pressure; a current detection means detecting current of the motor; a pressure control compensation means receiving detection results from each detection means and controlling the motor based on the detection results and a pressure instruction value to control pressure; and a buffer tank storing discharged fluid. A pressure control method for the compressor in the compressor unit comprises: an on-off control step 8 for turning off the motor when the pressure reaches a pressure upper limit value and turning on the motor when the pressure reaches a pressure lower limit value; and a rotation speed change control step 9 for changing rotation speed of the motor based on the flow rate used for the compressor. The pressure control compensation means selectively performs a control step out of two control steps 8, 9 based on tank capacity, the flow rate, and the current.