PURPOSE: To improve the ion collection efficiency, by operating a sodium ion detector under a negative pressure less than atmospheric pressure.

CONSTITUTION: Leaked sodium from a Na tube 13 of high speed breeder is carried by sample gas and a flow rate is controlled by a flow rate control valve 10 and also, pressure is reduced by the high pressure resistance of the valve 10. Hereby, a sodium ionization detector 2 connected to a pump 9 through a cutoff valve 11 is operated under the inside condition of 150W380Torr. negative pressure range by atmospheric pressure. Accordingly, a mean degree of freedom of a molecule in the detector 2 is increased and a collecting rate of a Na ion of a collector 4 of the detector 2 is improved and a highly accurate ionization detection is carried out.