PURPOSE: To provide a lost foam mold for casting a joint in a drainage vertical tube which can surely, simply and integratively join the lost foam molds divided into each part.

CONSTITUTION: In the foaming synthetic resin-made lost foam mold for casting the joint in the drainage vertical tube providing a main body, upper vertical tube connecting hole, lower vertical tube connecting hole and at least one piece of side branched tube connecting hole, the main body 10 made of the lost foam mold is constituted with an expanding part 11 having a tapered fitting part 12 and a fitting projecting part 14 at the lower end part, and a tapered tube part 31 having a part 32 to be fitted so as to be fittable to the tapered fitting part 12 and a fitting groove 34 so as to be fittable to the fitting projecting part 14 at the upper part. Then, by fitting the fitting projecting part 14 into the fitting groove 34 and also, the tapered fitting part 12 into the part 32 to be fitted, the expanding part 11 and the tapered tube part 31 are integrally joined by the tapered fitting and the lost foam mold 1 is made to be completed.