PURPOSE: To indicate the degree of vacuum in a wide range precisely only by one discharge by measuring the discharge current in a vacuum valve to be measured after amplifying the current with a logarithmic amplifier and them indicating the measured result on an indicator.

CONSTITUTION: A discharge current detecting part 1 detects the current produced when magnetron discharge occurs in a vacuum valve to be measured. The detected value is amplified by a logarithmic amplifier composed of a feedback element D with logarithmic characteristic and an amplifier A and measured by a measuring circuit 6 and then the degree of vacuum found by the measuring circuit 6 is indicated by an indicator according to the correlation between the current and the degree of vacuum which has been previously found. Thus, discharge having influence to the degree of vacuum is limited only once to indicate the degree of vacuum in a wide range precisely.