A heating cooker comprising: a cavity which is for accommodating an object to be heated therein; and a steam generator disposed outside the cavity for supplying steam to the cavity, the steam generator including: a boiler comprising a boiler body which has a first side wall, a second side wall and a bottom wall, wherein the boiler has a water inlet, a drain outlet, and a steam discharge port, and generates steam by heating water; a water supply pipe connected to the water inlet for supplying water to the boiler; a drainage pipe which is connected to the drain outlet, extends to a drainage pipe outlet positioned below the drain outlet by way of a curved portion thereof having a shape projecting upward from the drain outlet, and drains water from an inside of the boiler; and a steam supply pipe connected to the steam discharge port for supplying the steam generated by the boiler to the cavity, wherein the bottom wall of the boiler has an inner bottom surface downwardly inclined from the first side wall toward the second side wall which faces the first side wall, and the drain outlet is formed lower than the water inlet, characterized in that the water inlet is formed in the first side wall in a vicinity of the inner bottom surface, in that the drain outlet is formed in the second side wall at a position in the vicinity of the inner bottom surface, in that the steam generator further includes two heater units (26, 27) both of which are disposed at positions higher than the water inlet and the drain outlet, but at heights different from each other, and in that a plurality of heat transfer fins are formed in the inside of the boiler body integrally with the boiler body by molding in such a state where the heat transfer fins are connected to the two heater units (26, 27) and extend downwardly. a cavity which is for accommodating an object to be heated therein; and a steam generator disposed outside the cavity for supplying steam to the cavity, the steam generator including: a boiler comprising a boiler body which has a first side wall, a second side wall and a bottom wall, wherein the boiler has a water inlet, a drain outlet, and a steam discharge port, and generates steam by heating water; a water supply pipe connected to the water inlet for supplying water to the boiler; a drainage pipe which is connected to the drain outlet, extends to a drainage pipe outlet positioned below the drain outlet by way of a curved portion thereof having a shape projecting upward from the drain outlet, and drains water from an inside of the boiler; and a steam supply pipe connected to the steam discharge port for supplying the steam generated by the boiler to the cavity, wherein the bottom wall of the boiler has an inner bottom surface downwardly inclined from the first side wall toward the second side wall which faces the first side wall, and the drain outlet is formed lower than the water inlet, characterized in that the water inlet is formed in the first side wall in a vicinity of the inner bottom surface, in that the drain outlet is formed in the second side wall at a position in the vicinity of the inner bottom surface, in that the steam generator further includes two heater units (26, 27) both of which are disposed at positions higher than the water inlet and the drain outlet, but at heights different from each other, and in that a plurality of heat transfer fins are formed in the inside of the boiler body integrally with the boiler body by molding in such a state where the heat transfer fins are connected to the two heater units (26, 27) and extend downwardly.