PURPOSE: To accurately measure the working pressure of an apparatus which requires a wide pressure range by automatically correcting the zero point of a low-vacuum measurement system when a high-vacuum measurement system measures a degree of vacuum below the lower measurement limit of the low- vacuum measurement system.

CONSTITUTION: When the indicated pressure of a B-A type ionization vacuum gauge 6 reaches set pressure below the lower measurement limit of a diaphragm type manometer 11, a B-A type ionization vacuum gauge power source 17 operates a relay 18 to break a contact 19 in a vacuum-degree measurement power source 16, and the output 22 of the diaphragm type manometer 11 is fed back to an automatic zero-point correcting circuit 21 through an amplifier 20 for automatic zero correction so that an output voltage is zero. Then, the power source 17 is turned off and etching gas is introduced. The relay 18 is initialized and the input to the amplifier 20 is held at the earth potential; and the automatic zero-point correcting circuit is placed in a relay operation state, namely, holds a constant under high vacuum all the time.