A vehicle power transmission device includes: an input shaft; a planetary gear type speed reducer disposed concentrically to the input shaft to reduce and output rotation input to the input shaft; and a differential gear device disposed adjacently to the input shaft in a shaft center direction, the differential gear device being rotationally driven by the speed reducer to transmit a drive force to a pair of axles disposed on the shaft center while allowing a rotational difference between the axles, the speed reducer including a sun gear that is fit with a shaft end portion of the input shaft in a relatively non-rotatable manner, the sun gear being prevented from relatively moving toward the differential gear device by an annular snap ring fit and attached to an annular snap ring groove formed on an outer circumferential surface of the shaft end portion of the input shaft on the differential gear device side of the sun gear, the differential gear device including a differential case having a cylindrical end portion formed on the sun gear side or a member fixed thereto that is extended to the outer circumferential side of the snap ring, the differential case having a radial distance of an annular gap formed between the cylindrical end portion or the member fixed thereto and the snap ring set smaller than a groove depth of the snap ring groove.