To provide a proportional counter tube 11 capable of reducing generation of a tail distribution part of wave-height distribution made by an X-ray for excitation.

A conductor 18 to make an X-ray transmit is arranged by making a spacing between the inner face of an X-ray incidence window 17 in the outer case 12. The conductor 18 is made to have the same potential as that of the outer case 12. An electric field strength in a region between the X-ray incidence window 17 and the conductor 18 is reduced compared with a case in which there is no conductor 18 since both have the same potential. The electrons which come out into a gas 13 from the X-ray incidence window 17 is recombined with ions, the electrons are reduced from moving in the direction of a cathode 14, and generation of the tail distribution part of wave height distribution formed by the X-ray for excitation is reduced.