A reducing gas generator generates reducing gas including ammonia. The generator includes a solid reductant and a heat-generating portion. The solid reductant is formed in a columnar shape. A cross-sectional surface of the solid reductant has a constant shape and is perpendicular to a central axis of the solid reductant. The heat-generating portion includes a heat-generating surface opposed to a lower surface of the solid reductant in a vertical direction thereof and in contact with an entire region of the lower surface, and a heating element that heats the heat-generating surface when energized, so that the solid reductant is heated and decomposed to generate the reducing gas.