A hydrogen production system comprising: a humidifier that is supplied with a process fluid containing carbon monoxide and that mixes the process fluid with steam; a reactor that allows the humidified process fluid output from the humidifier to react in the presence of a catalyst to transform the carbon monoxide in the process fluid into carbon dioxide; a first channel through which the humidified process fluid is supplied from the humidifier to the reactor; a second channel through which the high-temperature process fluid that has reacted in the reactor flows; a circulation channel through which excess water in the humidifier is circulated; a first heat exchanger, disposed at an intersection of the circulation channel and the second channel, for heat exchange between the high-temperature process fluid that has reacted in the reactor and the fluid circulated through the circulation channel; characterized by a first bypass channel branched off the circulation channel so as to bypass the first heat exchanger; a first flow control valve disposed in the first bypass channel to control the flow rate of the fluid fed to the first heat exchanger; a third channel through which makeup water is supplied to the humidifier, the third channel and the second channel intersecting at a plurality of positions; a plurality of third heat exchanger (11, 12, 13), disposed at each intersection of the third channel and the second channel for gradual heating of the makeup water flowing through the third channel by heat exchange with high-temperature reacted process fluid; a separator disposed in the third channel to separate the fluid flowing through the third channel into water and steam; a fourth channel through which the steam separated by the separator is supplied to the first channel; a fifth channel branched off the fourth channel to supply a portion of the steam flowing through the fourth channel to the humidifier; and a fourth flow control valve disposed in the fifth channel to control the amount of steam supplied to the humidifier, and wherein the degree of opening of the first flow control valve and the degree of opening of the fourth flow control valves are controllable on the basis of an atmospheric temperature in the humidifier. a humidifier that is supplied with a process fluid containing carbon monoxide and that mixes the process fluid with steam; a reactor that allows the humidified process fluid output from the humidifier to react in the presence of a catalyst to transform the carbon monoxide in the process fluid into carbon dioxide; a first channel through which the humidified process fluid is supplied from the humidifier to the reactor; a second channel through which the high-temperature process fluid that has reacted in the reactor flows; a circulation channel through which excess water in the humidifier is circulated; a first heat exchanger, disposed at an intersection of the circulation channel and the second channel, for heat exchange between the high-temperature process fluid that has reacted in the reactor and the fluid circulated through the circulation channel; characterized by a first bypass channel branched off the circulation channel so as to bypass the first heat exchanger; a first flow control valve disposed in the first bypass channel to control the flow rate of the fluid fed to the first heat exchanger; a third channel through which makeup water is supplied to the humidifier, the third channel and the second channel intersecting at a plurality of positions; a plurality of third heat exchanger (11, 12, 13), disposed at each intersection of the third channel and the second channel for gradual heating of the makeup water flowing through the third channel by heat exchange with high-temperature reacted process fluid; a separator disposed in the third channel to separate the fluid flowing through the third channel into water and steam; a fourth channel through which the steam separated by the separator is supplied to the first channel; a fifth channel branched off the fourth channel to supply a portion of the steam flowing through the fourth channel to the humidifier; and a fourth flow control valve disposed in the fifth channel to control the amount of steam supplied to the humidifier, and wherein the degree of opening of the first flow control valve and the degree of opening of the fourth flow control valves are controllable on the basis of an atmospheric temperature in the humidifier.