A method for treating drain in hydrogen production includes steps of gasifying in a gasifier (1), reforming in a reformer (2), gas-liquid separation in a gas-liquid separator (4), PSA gas separation in a PSA separator (5) and evaporation in a drain treatment unit (6). In the gasifying, a mixed material containing methanol is heated and gasified. In the reforming, reformed gas containing hydrogen is produced from the mixed material by reforming reaction of methanol. In the gas-liquid separation, a liquid component is separated from the reformed gas and discharged as drain. In the PSA gas separation, hydrogen-rich gas and offgas are extracted from the reformed gas by PSA separation using an adsorption tower. In the gasifying, the offgas is burned, and the mixed material is heated by using the combustion gas as heat source. In the evaporation, drain is evaporated using the combustion gas after heating the mixed material as heat source.