PURPOSE: To obtain sure control at proper timing by estimating a pressure value at a succeeding time point on the basis of the pressure at the present or a preceding time point and comparing the estimated value with the set pressure value of the upper or lower limit for controlling the operation of a compressor.

CONSTITUTION: A pressure sensor 6 is connected to a control unit 7 with a microcomputer built-in, which is connected to a power source 2 of a compressor 1. The pressure signal continuously sent from the pressure sensor 6 is read in the control unit 7 as a pressure value at every fixed time width. Then, fixed calculations and comparisons of numerical values are carried on, and on the basis of results thereof, the power source 2 is predictively controlled for sure.