Abstract: A method and apparatus for indicating the contact structure of an electromagnetic contactor on the housing thereof using laser light is disclosed. The contact structure of a contactor to be marked with terminal numbers or the like is detected by a checking device. Based on the results of checking, a mask selector selects a suitable mask and positions it in the optical path of a laser. Laser light is shined through the mask onto the housing of the contactor. The laser light which passes through the mask and strikes the housing produces discoloration of the housing, and extremely clear terminal numbers, name plates, or other information in the form of characters, symbols, or shapes can be marked on the housing with the laser light. In addition to determining the contact structure, the checking device determines whether the contacts are faulty.