NOVELTY - A hydrogen generator (G1) comprises a reformer (a1) for conducting reforming reaction using a material containing an organic compound comprising at least carbon atom and hydrogen atoms, steam and reforming catalyst; a material supply portion (b1) to supply the material to (a1); a water supply portion (c1) to supply water to (a1); a heater to heat (a1); and a controller to control supply of the material from (b1) and supply of the water from (c1).    USE - In fuel cell system (claimed) for generating hydrogen-rich gas; for generating hydrogen supplied to hydrogen consumption equipment such as fuel cells.    ADVANTAGE - The generator exhibits high hydrogen generation efficiency and high reliability; is capable of reducing the start time depending on the temperature condition of (G1) at the start of the start-up operation and of increasing a temperature of the water evaporator while inhibiting excessive temperature increase in a reforming catalyst layer of the reformer. The second reference temperature is such that catalytic activity of the reforming catalyst layer is not degraded under absence of the steam.    DETAILED DESCRIPTION - A hydrogen generator (G1) comprises a reformer (a1) configured to conduct reforming reaction using a material containing an organic compound comprised of at least carbon atom and hydrogen atoms, steam and a reforming catalyst to generate hydrogen; a material supply portion (b1) to supply the material to (a1); a water supply portion (c1) configured to supply water to (a1); a heater configured to heat (a1); and a controller configured to control supply of the material from (b1) and the supply of the water from (c1).    (a1) Includes a water evaporator configured to evaporate the water supplied from (c1), a reforming catalyst layer (A) formed by the reforming catalyst, and a reforming temperature sensor to detect temperature of (A).    The controller includes a determination portion (d1) to determine whether or not the water evaporator has a temperature at which the evaporator can generate the steam based on the temperature of (A) detected by the reforming temperature sensor, and a supply control portion (e1) to control at least the supply of the water from (c1) to (a1) based on determination of (d1). (d1) Is configured to perform a first determination process (P1) in such a manner that (d1) compares the temperature of (A) detected when the heater starts heating (a1) to start a start-up operation of (G1) to a first reference temperature; and to perform a second determination process (P2) in such a manner that (d1) compares the temperature of (A) which is detected while (a1) is heated after (P1) to a second reference temperature which is higher than the first reference temperature when the temperature of (A) is not higher than the first reference temperature in (P1). (e1) Is configured to start the supply of water to (a1) when (d1) determines that the temperature of (A) is higher than the first reference temperature in (P1) or when (d1) determines that the temperature of (A) is higher than the second reference temperature in (P2).    INDEPENDENT CLAIMS are included for the following:    (1) operating (G1) involving: performing (P1) in such a manner that (d1) compares temperature of (A) detected when the heater starts heating (a1) to start a start-up operation of (G1) to a first reference temperature; performing (P2) in such a manner that (d1) compares the temperature of (A) which is detected while (a1) is heated after (P1) to a second reference temperature which is higher than the first reference temperature when the temperature of (A) is not higher than the first reference temperature in (P1); and starting the supply of the water from (c1) to (a1) when it is determined that the temperature of (A) is higher than the first reference temperature in (P1) or when it is determined that the temperature of (A) is higher than the second reference temperature in (P2); and    (2) a fuel cell system comprising: (G1); an air supply device; and a fuel cell configured to cause hydrogen supplied from (G1) and air supplied from air supply device to react to generate electric power.