A fuel cell system comprising: a plurality of reformers (Rn, R1-R3) for reforming a raw material to generate a reformed gas containing hydrogen; at least one fuel cell stack (F0, Fn, F1-F3)for generating electric power using the reformed gas supplied from the reformers; heat supply devices (Bn, B1-B3) provided as corresponding to the reformers (Rn, R1-R3), respectively, each of the heat supply devices (Bn, B1-B3) being configured to combust the reformed gas to supply heat to corresponding one of the reformers (Rn, R1-R3); and a mutual use line configured to supply the reformed gas generated in each of the reformers (Rn, R1-R3) to the heat supply device (Bn, B1-B3) corresponding to another one of the reformers, wherein wherein the fuel cell stack includes a plurality of fuel cell stacks (F0, Fn, F1-F3) corresponding to the plurality of reformers (Rn, R1-R3), respectively, the fuel cell system further comprising: a reformed gas supply line (12-n, 12-1, 12-2, 12-3) connecting the plurality of reformers (Rn, R1-R3) to the plurality of fuel cell stacks (F0, Fn, F1-F3), respectively, in one-to-one correspondence, a controller (Ct) configured to perform control in such a manner that when at least one of the reformers (Rn, R1-R3) is activated with another reformers being deactivated, and at least one of the another reformers being deactivated should start to be activated, the reformed gas generated in the reformer being activated is supplied through the mutual use line, to the heat supply device (Bn, B1-B3) corresponding to the reformer which should start to be activated to warm-up the reformer which should start to be activated by using the heat supply device (Bn, B1-B3), and then the reformer which should start to be activated is activated and the reformer being activated is deactivated, wherein the mutual use line is an off-gas mutual use line configured to supply a portion of an off-gas to the heat supply device (Bn, B1-B3) corresponding to another one of the reformers (Rn, R1-R3), the off-gas being the reformed gas supplied from each of the reformers to the fuel cell stack and discharged from the fuel cell stack. a plurality of reformers (Rn, R1-R3) for reforming a raw material to generate a reformed gas containing hydrogen; at least one fuel cell stack (F0, Fn, F1-F3)for generating electric power using the reformed gas supplied from the reformers; heat supply devices (Bn, B1-B3) provided as corresponding to the reformers (Rn, R1-R3), respectively, each of the heat supply devices (Bn, B1-B3) being configured to combust the reformed gas to supply heat to corresponding one of the reformers (Rn, R1-R3); and a mutual use line configured to supply the reformed gas generated in each of the reformers (Rn, R1-R3) to the heat supply device (Bn, B1-B3) corresponding to another one of the reformers, wherein wherein the fuel cell stack includes a plurality of fuel cell stacks (F0, Fn, F1-F3) corresponding to the plurality of reformers (Rn, R1-R3), respectively, the fuel cell system further comprising: a reformed gas supply line (12-n, 12-1, 12-2, 12-3) connecting the plurality of reformers (Rn, R1-R3) to the plurality of fuel cell stacks (F0, Fn, F1-F3), respectively, in one-to-one correspondence, a controller (Ct) configured to perform control in such a manner that when at least one of the reformers (Rn, R1-R3) is activated with another reformers being deactivated, and at least one of the another reformers being deactivated should start to be activated, the reformed gas generated in the reformer being activated is supplied through the mutual use line, to the heat supply device (Bn, B1-B3) corresponding to the reformer which should start to be activated to warm-up the reformer which should start to be activated by using the heat supply device (Bn, B1-B3), and then the reformer which should start to be activated is activated and the reformer being activated is deactivated, wherein the mutual use line is an off-gas mutual use line configured to supply a portion of an off-gas to the heat supply device (Bn, B1-B3) corresponding to another one of the reformers (Rn, R1-R3), the off-gas being the reformed gas supplied from each of the reformers to the fuel cell stack and discharged from the fuel cell stack. a reformed gas supply line (12-n, 12-1, 12-2, 12-3) connecting the plurality of reformers (Rn, R1-R3) to the plurality of fuel cell stacks (F0, Fn, F1-F3), respectively, in one-to-one correspondence, a controller (Ct) configured to perform control in such a manner that when at least one of the reformers (Rn, R1-R3) is activated with another reformers being deactivated, and at least one of the another reformers being deactivated should start to be activated, the reformed gas generated in the reformer being activated is supplied through the mutual use line, to the heat supply device (Bn, B1-B3) corresponding to the reformer which should start to be activated to warm-up the reformer which should start to be activated by using the heat supply device (Bn, B1-B3), and then the reformer which should start to be activated is activated and the reformer being activated is deactivated, wherein the mutual use line is an off-gas mutual use line configured to supply a portion of an off-gas to the heat supply device (Bn, B1-B3) corresponding to another one of the reformers (Rn, R1-R3), the off-gas being the reformed gas supplied from each of the reformers to the fuel cell stack and discharged from the fuel cell stack.