The present invention provides a flow soldering apparatus that can suppress the occurrence of soldering defects, even if VOC-free flux or low-VOC flux is applied to an electronic circuit substrate. In other words, the flow soldering apparatus according to the present invention includes: a water content sensor for measuring a residual water content of a surface of a substrate; and a monitoring device for judging whether soldering quality is satisfactory or not by using a residual water content value of the surface of the substrate acquired from the residual water content measured by the water content sensor, on the basis of a correlation between a residual water content value of a surface of a sample substrate and a residual water content value of through holes of the sample substrate, as acquired previously by using a plurality of substrate samples.