PURPOSE:To measure the concentration of underground radon, by performing a measurement of alpha ray of underground air from an underground gas hole and a background measurement with the introduction of fresh air to remove the background varying by effect of a radon daughter nuclide or the like. CONSTITUTION:A measuring device 100 comprising a detector 14 and a scalar 18, a passage tube for passing underground air from a pump 16 and an underground gas hole 10 and, moreover, a control circuit 20 for automatically controlling an air passage mechanism are arranged and a passage tube is disposed from a fresh air intake port 22 to the detector 14. Measurement comprises cleaning of the inside of the detector passage tube by a fresh circulation, measurement of a back ground, a constant reduction in pressure within the detector passage tube, a constant amount suction of the underground air and measurement of the concentration of radon. In the measurement of the background, all valves A, B and C are closed to remove a radon daughter nuclide thereby enabling the obtaining of a stable measured value.
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