PURPOSE: To solder objects to be soldered in a short time while maintaining the accuracy of assembling, by sandwiching the objects to be soldered with two heat source bodies heated to specified temps., lifting the objects from a jig, and returning the objects again on the jig and cooling the same.

CONSTITUTION: An assembly jig on which assembled parts 1, 2, 3 are placed are put in the positional relation of two heat source bodies 4, 5. The bodies 4, 5 are beforehand controlled to a suitable temp. The body 5 descends to press and heat the part 3; at the same time, guides 7, 8 on the assembly jig escape to the right and left. The body 4 ascends to sandwich the parts 1, 2, 3 between the same and the body 5, lifts the parts as they are from the jig, and holds the same for an adequate time as they are to melt solder 2. Then the body 4 descends, the parts 1, 2, 3 return to the jig, the solder 2 is solidified as it is deprived of heat by the jig and the body 5 ascends.