A reforming catalyst is composed of a plurality of catalyst layers arranged in series, and when started from a low temperature, preheated air is supplied in parallel to a point upstream of each catalyst layer, thereby the catalyst in each layer is self-heated simultaneously so that the catalyst is heated. After the catalyst is heated, feed of air is temporarily stopped, and supply of fuel gas is started. Next, air is supplied while controlling its flow rate so that temperatures in the catalyst do not exceed the temperature that the catalyst can withstand. Thus, no other special equipment is needed, and the catalyst can be started quickly (cold start). The catalyst can be made compact enough to install it in a electric vehicle.