PURPOSE: To detect the degree of vacuum at the inside of a receptacle to be detected, by installing a vacuum receptacle so that it is connected to gas in the receptacle to be detected and a means for producing a gas molecule in the vacuum receptacle to measure the degree of vacuum at the inside of the vacuum receptacle.

CONSTITUTION: An outer wall of a pressure-sensitive part 10 of a vacuum detector 9 forms an airtight vacuum receptacle which is connected to a vacuum receptacle 8 at a vacuum valve side and is secured airtightly against the air. A high voltage power source 11, applying a high voltage between conductors separated from each other, of the pressure-sensitive part 10 has one end, linked to a moving electrode 4, and the other end coupled to an electrode 18 of the pressure-sensitive part 10 through a current detector 12. An electrode 23 of the pressure-sensitive part 10 is joined to a flange 2. An AC voltage 14 is energized to a coil 13, and an AC magnetic field is generated at the inside of an insulating body 16 to heat a conductor fine wire assembly body 25. The rise in the temperature of the assembly body 25 produces a gas molecule and causes an insulation damage between the electrodes 18 and 23 to detect it by means of the device 12. The measurement of a current vacuum pressure permits the detection of the degree of vacuum that precedes the heating of the assembly body 25.