PURPOSE: To decrease soldering failures by applying flux to a soldering necessary part while performing dehydrating treatment by a dehydrating agent.

CONSTITUTION: In an outer tank 1 of a flux applying device, a lateral triangular pyramid-shaped nozzle 2 is provided vertically to the carrying direction of a printed board. In the lower part of the nozzle 2, a porous air blow-off body 4 is placed. This flux applying device is provided with a dehydrating agent treatment device 10, too, and by a pump 11, the flux contacts a dehydrating agent, its moisture is removed, and it is fed back to the outer tank 1. According to this mechanism, the flux is applied to the printed board under an operation of dehydration by the dehydrating agent treatment device 10, therefore, the moisture quantity of the flux is always kept constant. Accordingly, soldering failures based on remaining, scattering, etc. of the moisture are decreased remarkably.