A method for manufacturing a vehicle power transmission device having a case and a ring gear fitted to the outer peripheral surface of the case to transmit drive power from a drive source. The method includes: a welding step in which an annular flange is provided on the case or the ring gear and butt welding is performed by lapping a weld bead along the contact portion of one side surface of the annular flange and the case or the ring gear; a measurement step in which the outline of the other side surface of the annular flange placed in close proximity to the one side surface are measured; and an evaluation step in which a weld condition is evaluated based on part of the outline, which is determined in the measurement step, of a portion corresponding to the lapping portion of the weld bead.