PURPOSE: To eliminate the need for new heat sources hence controllers for the same and to simplify a soldering device in soldering of printed wiring substrates by using the heat of molten solder in heating the flux to be supplied to spray nozzles.

CONSTITUTION: A printed wiring substrate 21 mounted with electronic parts 22 is moved in an arrow direction toward the top surface of the jet waves (b) of the molten solder (a) of a soldering tank 1 after preheating. Just before the substrate 21 contacts with the waves (b), a spraying device 11 is operated to spray the heated mist-like flux (c) from each spray nozzle 13 toward the bottom surface of the substrate 21. Thereafter the substrate 21 is brought into contact with the waves (b) to solder the lead wires 23 of the parts 22. During the time just before the substrate 21 detaches from the waves (b) until the same detaches completely therefrom, a spray device 12 is operated and the flux is sprayed in the same manner as mentioned above. The liquid flux is heated in a tank 16 which is mounted on the side surface of the tank 1 and to the flux contained in which the heat of the solder (a) conducts. The heated flux is supplied to the nozzles 13.