A method of producing hydrogen, comprising the steps of: heating fuel in presence of steam; converting the steam and carbon monoxide of a produced gas obtained in the heating step into carbon dioxide and the hydrogen by causing the steam and the carbon monoxide to react with each other; cooling the produced gas after the converting step and causing the produced gas to contact a carbon monoxide selective oxidation catalyst to generate a reformed gas; and separating the hydrogen from the reformed gas by a gas separator, wherein: the carbon monoxide selective oxidation catalyst is being cooled by cooling water; heat energy recovered by the cooling water having cooled the carbon monoxide selective oxidation catalyst is utilized to generate the steam supplied in the converting step; and the gas separator is an alkali absorption device configured to absorb the carbon dioxide by an alkali absorption method, in which: by pressurizing an absorbing liquid and supplying the absorbing liquid from an absorption tower to a regeneration tower, an inside of the regeneration tower is maintained at about critical pressure of the carbon dioxide or at not lower than the critical pressure of the carbon dioxide, and when the absorbing liquid is supplied from the regeneration tower to the absorption tower, the absorbing liquid is depressurized by a pressure regulator. heating fuel in presence of steam; converting the steam and carbon monoxide of a produced gas obtained in the heating step into carbon dioxide and the hydrogen by causing the steam and the carbon monoxide to react with each other; cooling the produced gas after the converting step and causing the produced gas to contact a carbon monoxide selective oxidation catalyst to generate a reformed gas; and separating the hydrogen from the reformed gas by a gas separator, wherein: the carbon monoxide selective oxidation catalyst is being cooled by cooling water; heat energy recovered by the cooling water having cooled the carbon monoxide selective oxidation catalyst is utilized to generate the steam supplied in the converting step; and the gas separator is an alkali absorption device configured to absorb the carbon dioxide by an alkali absorption method, in which: by pressurizing an absorbing liquid and supplying the absorbing liquid from an absorption tower to a regeneration tower, an inside of the regeneration tower is maintained at about critical pressure of the carbon dioxide or at not lower than the critical pressure of the carbon dioxide, and when the absorbing liquid is supplied from the regeneration tower to the absorption tower, the absorbing liquid is depressurized by a pressure regulator. the carbon monoxide selective oxidation catalyst is being cooled by cooling water; heat energy recovered by the cooling water having cooled the carbon monoxide selective oxidation catalyst is utilized to generate the steam supplied in the converting step; and the gas separator is an alkali absorption device configured to absorb the carbon dioxide by an alkali absorption method, in which: by pressurizing an absorbing liquid and supplying the absorbing liquid from an absorption tower to a regeneration tower, an inside of the regeneration tower is maintained at about critical pressure of the carbon dioxide or at not lower than the critical pressure of the carbon dioxide, and when the absorbing liquid is supplied from the regeneration tower to the absorption tower, the absorbing liquid is depressurized by a pressure regulator. by pressurizing an absorbing liquid and supplying the absorbing liquid from an absorption tower to a regeneration tower, an inside of the regeneration tower is maintained at about critical pressure of the carbon dioxide or at not lower than the critical pressure of the carbon dioxide, and when the absorbing liquid is supplied from the regeneration tower to the absorption tower, the absorbing liquid is depressurized by a pressure regulator.