PURPOSE: To enable the pressure of sealed gas to be measured easily and with high accuracy, by cold lighting a fluorescent lamp with a stabilizer constituted by a resonant stabilizer having a secondary no-load voltage waveform in which resonant waves are superposed but having no phase advancing capacitor, and obtaining time values at predetermined points in the tube voltage waveform.

CONSTITUTION: A stabilizer 2 is constituted by a resonant stabilizer having a secondary no-load voltage waveform where resonant waves are superposed, but without a phase advancing capacitor. A fluorescent lamp 1 is lit with an input voltage held constant at 70V. In the tube voltage waveform 6 thus indicated by a detector 3, the time T1 from the point 7 where the waveform 6 intersects the line of zero volt to the point 8 where the peak voltage is obtained for the first time, the time T2 from the point 7 to the point 9 where an arbitrarily set voltage V1 is obtained after passing the first peak voltage, and the time T3 from the point 7 to the point 10 where the lowest voltage is obtained after the first peak voltage. Respective relations between the pressure of sealed gas and the values of T1, T2 and T3 on the tube voltage waveform are previously obtained so that the pressure of the sealed gas can be obtained very easily and with high accuracy.