To provide a proportional counter tube 11 with the lower measurement limit lowered of an element wanted to measure by improving the occurrence of tail distribution in pulse-height distribution made by radiation for excitation.

A gas 13 to be sealed up inside an envelope 12 contains 30 to 90% of helium in terms of a pressure ratio. Containing 30% or more helium in terms of a pressure ratio reduces detection sensitivity on radiation for excitation while lowering tail distribution in pulse-height distribution made by the radiation for excitation. Helium of 90% or less in terms of a pressure ratio maintains detection sensitivity on radiation of an element to be measured.