For a reforming device that generates fuel gas for fuel cells by decomposing hydrocarbon compounds such as natural gas and then using a hydrogen separation composite to selectively transmit hydrogen, a hydrogen separation composite having the following structure is used. A porous support medium made of ceramics, etc. is formed, and a hydrogen separation metal is supported in the pores so as to fill the inside of the support medium. It is also possible to support a reforming catalyst. By doing this, it is possible to increase the area at which the hydrogen separation metal contacts gas, so the hydrogen transmission performance is increased. Furthermore, to prevent raw material gas leaks due to pin holes, high pressure gas is supplied to the hydrogen extraction side, and the total pressure is made higher than the pressure on the raw material gas supply side without making the hydrogen partial pressure higher. By using these means, it is possible to increase the hydrogen separation performance for the reforming device and to make the device more compact.