In a tubular body mounting step, a tubular body is connected to a first end constituting member and a second end constituting member of a fuel cell stack, respectively. At this time, a first opening and a second opening formed at both ends of the tubular body are closed by the first end constituting member and the second end constituting member, respectively. Accordingly, the fuel cell stack is surrounded by the tubular body. A gap is formed between the outer wall of unit cells of the fuel cell stack and the inner wall of the tubular body. In a determination step, a test gas is supplied into the fuel cell stack. Further, whether the test gas exists in the gap is determined.