A fuel reforming system comprises a reforming catalyst section (4) performing reformate reactions on supplied fuel gas, a membrane hydrogen separator (5) extracting hydrogen from the reformate gas, a supply device (8 14) supplying combustion gas for heating the membrane reactor (2), a sensor (16) for detecting the temperature of the membrane hydrogen separator (5) and a controller. The supply device supplies combustion gas to the membrane reactor (2) before the reforming catalyst section starts reformate reactions during startup of the membrane reactor (2). Furthermore the supply device stops supply of combustion gas to the membrane reactor (2) when the temperature of the membrane hydrogen separator (5) is greater than or equal to a target temperature. Thereafter the fuel supply device supplies fuel to the fuel reforming catalyst section (4). In this manner, hydrogen embrittlement in the membrane hydrogen separator can be avoided.