A manufacturing method of a supporting structure for vehicle has steps of preparing a casting mold having: a first and a second casting mold, which are matched to each other at casting, and has forming faces which form at least sides of first and second fixing portions and the connecting portion and a third casting mold having a forming face which forms a fixing face of the second fixing portion and a protruding body which forms a recess hole penetrating to an interior of the connecting portion from the fixing face of the second fixing portion; matching the third casting mold together with the first and the second casting mold so as to form a cavity for integrally forming the first fixing portion, the second fixing portion and the connecting portion in the casting mold; and pouring molten metal into the cavity so as to mold the supporting structure.