PURPOSE: To make a solder coat formed on a surface pad uniform by making fusing conditions of a solder plating layer constant.

CONSTITUTION: A fusing device 12 is provided, which is provided with a suction duct 2-2 for sucking indoor air to a furnace body 2-1 wherein a heat source 2-4 whose heat generation amount is controlled by a temperature sensor 2-5 and an exhaust duct 12-3 having means 12-3a, 12-3b for controlling an amount of air sucked by the suction duct 2-2. Environment conditions of atmospheric gas supplied to the fusing device 12 is controlled to a certain value, an amount and a temperature of ventilating gas inside the furnace body 2-1 are controlled by control means 12-3a, 12-3b and a temperature sensor 2-5 and a printed wiring substrate 1 is inserted; thereby, a solder plating layer applied to a surface pattern and a surface pad of the printed wiring substrate 1 is fused and a solder film is formed.