PURPOSE: To increase productivity, etc. by providing an ejector type mixer on the primary side of a reactor tube to increase the pressure of raw material gas in the system and a preheater which utilizes waste heat of formed gas and preheat the raw material gas.

CONSTITUTION: A raw material hydrocarbon such as LPG is fed into a raw material evaporator 24 by a raw material pump 23 for evaporation. The gas is superheated in a superheater 25, desulfurized in a desulfurization tower 26 and suctioned by an ejector 27 which employs a high-pressure process steam as driving fluid, for conversion to a high-pressure mixed raw material gas. The mixed gas is preheated in a raw material preheater 28 and fed into a reactor tube 21 for reaction. The preheated gas is subjected to heat exchange with mixed raw material gas in the preheater 28 and the amount of CO is reduced in a modifier 29. The modified formed gas is subjected to heat exchange in the evaporator 24, compressed in a compressor 30 for high pressure and is stored in a spherical tank 31.