A hydrogen generator comprises a reformer which generates a hydrogen-containing gas from a steam and a material gas, a first gas supply device which supplies the material gas, a combustor which combusts an exhaust gas exhausted from the reformer to heat the reformer, a combustion air supply device which supplies air to the combustor, a second gas supply device which supplies another gas different from the material gas to the reformer or to a passage connecting the reformer to the combustor, and a controller. The controller is configured to control the combustion air supply device to increase an amount of the air supplied to the combustor (S103), in association with start of supply of the another gas from the second gas supply device (S104), in a state where the material gas is supplied from the first gas supply device to the reformer and the exhaust gas is combusted in the combustor (S101).