An add-on control circuit device for insertion into electronic apparatus such as a laser printer having a first microprocessor with an address line connected to an add-on card connector, a ROM that stores programs for the microprocessor, and a data converter that provides data in the form of memory addresses. The add-on circuit uses a printed circuit board of mounted inside of an add-on cartridge or casing, made from high thermal transfer material such as aluminum. When the cartridge is inserted into a printer, the processor inside of the printer reads identification data stored on a ROM inside of the cartridge and conducts processing according to a specified processing program in response to the identification data. The microprocessor is located in a mounting area near the front end of the cartridge, when viewed along the direction of insertion, so that the microprocessor is more effectively cooled by the flow of cooling air inside of the printer. Other embodiments incorporate cooling structures, thermal transfer materials disposed between heat generating elements and an outside casing, and ventilation holes.