PURPOSE: To control smoothly working pressure for high accuracy polishing in a working pressure control device used for polishing wafers or the like by using an apparatus for continuously changing the pressure to gradually increase or decrease the working pressure relative to time axis.

CONSTITUTION: To gradually increase working pressure of a pair of surface tables 10, 11 to shift a polishing device from the pressure reducing working to the own weight one, the set voltage of a desired value setter 21 in a control device 20 is altered from Va to zero, the down signal is sent from a comparator 23 to an up-down counter 26 so that the counter 26 is subjected to substraction by pulses from a voltage frequency converter 25 according to which the control signal Vt is reduced and secondary pressure P2 in a lower cylinder chamber 13b is reduced toward zero to increase the working pressure of a upper surface table 10. The rate of increasing pressure determines pulse frequency from the converter 25 by the value of set voltage VR1 of an inclination setter 24 in the control device 20 to gradually increase with a proper gradient the working pressure reaching from the working pressure in the pressure reducing working to that in the own weight working.