To provide an automatic soldering system which can effectively solder a necessary part alone while preventing flaw and deformation of a solder applied body.

An automatic soldering system comprises a flux application device, a jig assembly device 51, a flow solder bath, a cooling device and a jig disassembly device, etc. An assembly stage 58 of the jig assembly device 51 is provided with a side part supporting plate 68 which positions three plates of a lead frame 1 and both thin plate jigs 21, 31 mutually. A vacuum chuck which is a first transfer means transfers the three plates onto the assembly stage 58. A cassette slider 53 which is a second transfer means transfers a cassette-like jig 41 from a jig stocker 57 to the assembly stage 58 and the three plates are contained in the jig 41. A containment auxiliary means 67 holds the positioning state of the three plates until containment operation is finished. Thereafter, an assembled jig unit passes through a molten solder supply process, etc.