PURPOSE: To realize a board of excellent soldability by a method wherein the upside of a warpage preventive plate in a solder tank is made to abut firmly against the underside of the board to prevent it from warping downward to become convex state as both of the sides of the board are held by two or more feed pawls.

CONSTITUTION: The upside 38a of a warpage preventive plate 38 provided in a soldering tank is made to butt against the underside 2a of the board 2 as both the sides 2a and 2b of a board 2, on which an electronic component 1 is mounted, transferred on a conveyor are held by two or more feed pawls 5 when the board is soldered. The force which makes the board 2 warp downward to become convex state is supported by the warpage preventive plate 38 to prevent the board 2 from warping, and the upside 38a of the warpage preventive plate 38 is made to sink into a molten solder 4 while an oxide removing device is in operation. By this setup, an excellent soldability can be realized.