PURPOSE: To make easy the operation and maintenance and to increase the accuracy, by balancing the output pressure and the set weight balance.

CONSTITUTION: The differential transformer 4 is placed at the external circumference of the rotor meter 1, and it is connected to the servo valve 7 via the amplifier 6. Thus, when the output pressure P is fed to the rotor meter 1, the float 12 is lifted up, deviation voltage is produced in the differential transformer 4, and after amplification, it is inputted to the servo valve 7 and the air pressure from the supply air source 8 is reduced, the output pressure P is reduced, and when the float 12 is lifted down, the opposite operation is made. The float 12 is made stable at the set position to form the reference output pressure corresponding to the weight 3 automatically. The pressure can comparatively long time be kept to achieve desired purpose.