[en] At the J-PARC Hadron Experimental Facility, a gold target is bombarded with intense 30-GeV protons to produce secondary particles provided for various physics experiments. The newly-installed gold target is placed in a hermetic chamber, and to monitor the soundness of the target, the chamber is filled with circulating helium gas, whose radioactivity is continuously monitored with a Ge detector and a NaI(Tl) detector. Various γ-emitting nuclides were detected in the helium gas in beam-operation conditions. The relationship between beam operation and radionuclides production in helium gas was investigated. Radionuclides such as C-10, O-19, O-20, F-20, Ne-23, Ne-24, Na-24 (daughter of Ne-24), Ar-41, and annihilation peak, were detected in relatively low 24-kW beam power operations. These nuclides are produced in the beam windows and/or the gold target and transferred into the helium gas phase as gaseous chemical species. On the other hands at higher beam intensity of 42-kW, mercury isotopes, such as Hg-191m, Hg-192, Hg-193m, Hg-195, Hg-195m, and Au-192 as daughter of Hg-192 were detected in the helium gas. These detected nuclides are produced in the gold target and transferred into the helium gas phase as mercury vapor. Dependence of the counting rates for mercury isotopes on the beam power implies that transferring ratio of mercury isotopes into the gas phase was governed by the surface temperature of the gold target. Further detailed analyses are in progress to understand the behavior of induced radionuclides in gas phase. (author)