PURPOSE: To obtain a specific gas amplifier by forming at least a portion of an enclosed housing provided with an incident window for radiation with movable wall face for varying the inner capacity while adjusting the gas density easily after manufacture.

CONSTITUTION: For example, a tubular auxiliary chamber 7 is provided at a portion of housing 1 to fasten a screw 8 onto the bottom face while a bellows 9 is contained to bond one end to the bottom face of housing 1 thereafter the bellows 9 and housing 1 are communicated through a hole 10 to bond the leading edge of the screw 8 rotatably to the bottom plate 11 fixed to the other end of bellows 9. Upon rotation of screw 8, the bellows 9 is expanded or compressed to vary the inner capacity of housing 1 thus to set the density of electrolytic dissociation gas at desired level.