This study evaluated the hydrogen issue in the initial operation of a filtered containment venting system (FCVS). We calculated the volumetric concentration of hydrogen, steam, and air in the postulated FCVS connected with the OPR 1000, as a target nuclear power plant, under a station blackout using the MELCOR computer code (version 1.8.6). A large amount of steam and a flammable mixture generated during a severe accident are immediately released from the containment building to the FCVS when the pressure in the containment building approaches a set value. The constituent ratio of the flammable mixture of hydrogen, steam, and air can change due to the different thermal-hydraulic conditions between those due to a severe accident in the containment building and the initial condition in the FCVS. The volumetric concentration of hydrogen was 6% in the containment building just before the operation of the FCVS. It increased up to 9% in the FCVS vessel during the early operation, and steam condensation occurred simultaneously. The atmospheric condition including steam, hydrogen, and air in the FCVS can enter the combustion zone in the Shapiro diagram.