PURPOSE: To carry out analysis without receiving the effect of background radioactivity or the oxide compound in off gas and generating the deterioration of an electrode by using electrodeless discharge plasma due to high frequency discharge in optogalvano-analysis.

CONSTITUTION: Off gas containing krypton being a substance to be measured is introduced into a discharge cell 2 by a vacuum pump 1. The high frequency power generated from a high frequency power supply 5 is transmitted to a discharge cell and the carrier gas introduced into the discharge cell 2 is ionized to generate an electron. This electron receives high frequency energy to be accelerated to excite krypton. A wavelength variable laser generating part 8 forms laser beam having the wavelength resonated with krypton and, when the discharge cell 2 is irradiated with the laser beam by an optical chopper 9, krypton in the discharge cell 2 is further excited to generate a change in the impedance within the discharge cell 2. This change is taken out by a coil 10 to be sent out to a lock-in amplifier 12 and a phase is detected to know the conc. of krypton on the display of an output meter 13.