A steam generator includes: a base module which has a water supply port for receiving steam water; and a steam module which is connected to an upper side of the base module, in which the steam module includes: a main body which has an upper surface and a lower surface; a flow pipe which is positioned at a central portion of the main body and into which the steam water flows; and a heater which supplies heat to the flow pipe, and in which another steam module is selectively coupled to an upper side of the steam module. According to this, the internal volume of the steam generator and the output of the heater provided in the steam generator can be selectively varied so as to provide an optimum output according to the volume of a cooking device in which the steam generator is installed.