A joining jig 5 is constituted of a pair of flat-plate sandwiching portions 17, 18 including sandwiching faces 15 which sandwich therebetween a platy member laminate 4 prepared by laminating two platy members 2, 3 with providing a solder on a joined face between the platy members. The joining jig has a plurality of vapor inflow ports provided in the sandwiching face 15 so that surplus solder vapor on the joined face flows into the vapor inflow ports in a case where the platy member laminate 4 is heated while sandwiching the platy member laminate between the pair; vapor flow paths provided so that the vapor flow paths communicate with the vapor inflow ports; and vapor discharge ports 42 provided in the side surface of the joining jig so that the vapor discharge ports communicate with the vapor flow paths to discharge the vapor of the solder to the outside.