A modular grill cooker that is easily installed in a multipurpose cooking apparatus, including an outer casing and an inner casing set in the outer casing. A heating unit is mounted to a heat reflecting plate fastened to the inner casing. A grilling unit is seated on a frame provided along upper edges of the inner and outer casings. An elastic piece is fitted over a projection on a back surface of the heating unit to mount the heating unit to the heat reflecting plate. A grease collecting unit is set in the inner casing to collect grease therein and reflect heat onto the grilling unit. First and second contact terminals are respectively provided on the outer casing and on the multipurpose cooking apparatus to supply electricity to the heating unit when the grill cooker is installed in the multipurpose cooking apparatus.