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 covering body 18 is installed by covering the inner face of the outer case 12 which the X-ray incident from an X-ray incidence window 17 of the outer case 12 reaches. The covering body 18 has a higher X-ray transmittance than that of a material quality of the outer case 12, and has a thicker thickness than a flight distance of electrons generated by obtaining energy of the X-ray. Even if the X-ray reaches the inner face of the outer case 12 that is the covering body 18, without being absorbed by a gas 13, the electrons generated in the covering body 18 are reduced from going out from the covering body 18 into the gas 13, and generation of the tail distribution part of wave-height distribution made by the X-ray for excitation is reduced.