Abstract: A pair of soldering irons are fixed to a sliding plate at a predetermined interval. The soldering irons are integrally moved so as to reciprocate in a rectangular direction relative to a conveyor belt. One of the soldering irons is conveyed to a working position of the conveyor belt and the other of them is separated from the conveyor belt. While one of the soldering irons solders a circuit board, the other is cleaned. The circuit board has a slit into which a metal plate is inserted and soldered. For the slit, a soldering land constituted of a main-land and a sub-land is provided. The main-land is formed along one of longer sides of the slit. The sub-land is elongated from the main-land along a shorter side of the slit. The soldering land is not formed all around the slit so that the slit is not closed by solder when the circuit board is dip-soldered.