NOVELTY - The mean molecular weight of the component of the source gas is calculated based on the ratio of the measured volumetric and mass flow rate of the source gas from respective flowmeters (9,12). A controller controls flow rate of water vapor or source gas to servo system based on calculated mean molecular weight and measured value of mass flow rate.    USE - For controlling hydrogen manufacturing plant which generates hydrogen used in fuel cell for electric power generation.    ADVANTAGE - Due to the controller the flow amount of water vapor or source gas is controlled accurately with respect to the composition and flow amount of source gas. Provides compact simple structure hydrogen manufacturing plant. Steam and carbon ratio is optimized.    DETAILED DESCRIPTION - The mass flow meter detects mass flow rate of the source gas corresponding to the change in heat amount. A thermal flow sensor (4) outputs signal corresponding to the change in heat amount. The controller carries out tracking control of the mass or volumetric flow rate of water vapor based on mass flow rate of source gas which is saturated hydrocarbon gas having portion containing substitution product. The controller corrects the calculated mean molecular weight. The source gas is indicated by specific formula. In the hydrogen manufacturing plant, source gas is made to react with water vapor so as to manufacture hydrogen which is supplied to fuel cell which generates electric power by making hydrogen to react with oxygen. The plant uses several steam deforming systems or chemical substance including system in which source gas is made to react water vapor. INDEPENDENT CLAIMS are also included for the following:    (1) hydrogen manufacturing apparatus; and    (2) hydrogen manufacturing method.    DESCRIPTION OF DRAWING(S) - The figure shows the hydrogen manufacturing connected to the fuel cell electric power generation system. (Drawing includes non-English language text).    thermal flow sensor (4)    desulfurizer (5)    source gas volumetric-flow-rate meter (9)    control arithmetic section (11)    water vapor flowmeter (12)