To provide a detection method for easily determining a quantity of 222Rn, but not directly measuring radiation of 222Rn, 210Pb, 210Bi or 210Po.

A hydrocarbon fluid containing a small amount of 222Rn is passed through an adsorption tower for radioactivity measurement and is brought into contact with an adsorbent with a mass of M (g) which the adsorption tower is filled with under the same pressure condition P (MPa) as the process or at a test adsorption pressure Pe (MPa), equilibrium adsorption of the 222Rn in the carbonization fluid is performed, the amount of γ dose discharged from 214Bi or 214Pb which is a decay product of 222Rn adsorbed in the adsorbent is measured, and the quantity of the 222Rn absorbed to the adsorbent is determined.