NOVELTY - A hydrogen generator equipped with a modification portion (1) which carries out modification reaction to produce a hydrogen-containing gas, a material-providing portion (3) which supplies raw material, a water supply portion (2), and a modification heating portion (5), is operated as follows. While starting, the raw material and water exist in the modification portion. Supply of raw material and water to the modification portion is carried out, and the supply is stopped after carrying out heating temperature-rising of the modification portion.    USE - For operating hydrogen generators in fuel-cell power-generating devices.    ADVANTAGE - The operating method reduces the carbon deposition by thermal decomposition of raw material during starting, and prevents pressure-drop buildup, reduction in catalytic activity, and gas-passageway obstruction. The starting time of apparatus is shortened and starting energy is reduced.    DETAILED DESCRIPTION - A hydrogen generator equipped with a modification portion (1) which carries out modification reaction of raw material and steam in the presence of a catalyst to produce a hydrogen-containing gas, a material-providing portion (3) which supplies the raw material containing an organic compound comprising carbon and hydrogen to the modification portion, a water supply portion (2) which water for producing steam to the modification portion, and a modification heating portion (5) which supplies heat required for modification reaction to the modification portion, is operated as follows. While starting, the raw material and water exist in the modification portion. Supply of raw material and water to the modification portion is carried out, and the supply is stopped after carrying out heating temperature-rising of the modification portion. INDEPENDENT CLAIMS are included for the following:    (1) hydrogen generator; and    (2) fuel-cell power-generating device.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the hydrogen generator. (Drawing includes non-English language text)    Modification portion (1)    Water supply portion (2)    Material-providing portion (3)    Modification-heating portion (5)    Transformation portion (6)