In a wave soldering tank using a conventional screw pump, pulsation sometimes occurred in which molten solder spouting from the discharge nozzle moved up and down. The cause of the occurrence of the pulsation in a screw pump was that the gap between the screw pump and the casing was wide, so there was reverse flow through the gap. It is conceivable to narrow the gap, but if the gap is made narrow, the screw pump ends up contacting the casing if the screw pump becomes eccentric.A wave soldering tank according to the present invention makes the diameter of the inlet in the bottom of the casing smaller than the diameter of the screw pump so that reverse flow does not take place even if the gap between the screw pump and the casing is wide. Flow is made more stable by providing outwardly flaring guide walls on the outlet of the casing.