An apparatus for removing flammable gas is started only when a concentration change in the flammable gas in a primary containment vessel is detected, so that the decrease in activity of a catalyst due to a catalyst poison is prevented. In one embodiment this apparatus has a catalytic recombining apparatus equipped with a catalyst (1) for reacting the flammable gas, a case (2) that houses the catalyst, and lids (4a and 4b) installed at openings (3a and 3b) of the case (2). Adhesive parts (5a and 5b) which adhere the lids (4a and 4b) and the case (2) is melted at high temperatures, and heating elements (6a and 6b) composed of an oxidizing catalyst of hydrogen is installed in contact with the adhesive parts (5a and 5b). Thus, the lids are not only closed under normal conditions but opened when the adhesive parts (5a and 5b) being melted along with the temperature increase in the surrounding atmosphere. In another embodiment, the apparatus makes use of a porous columnar body.