A motive power transmission device for a vehicle, comprising: a transmission configured to perform gear-shifting and to output motive power input from a drive source; and a drive shaft that is coupled with the transmission and configured to transmit the motive power from the drive source, which is output from the transmission, to a drive wheel, wherein the transmission has a transmission mechanism unit that is arranged in a transmission casing such that an axis line extends in a vehicle front-rear direction and that performs gear-shifting for the motive power from the drive source, the drive shaft extends in a vehicle width direction on a central side of the transmission mechanism unit in the vehicle front-rear direction and is provided such that the drive shaft passes through the transmission casing, and the transmission mechanism unit includes an input shaft and an output shaft that are arranged on the same axis line, a counter shaft that is arranged in parallel with the input shaft and the output shaft, and a transmission gear unit that has plural gear trains (G1 to G6) each of which is formed with a first gear (31 to 36) provided to the input shaft or the output shaft and a second gear (41 to 46) provided to the counter shaft and meshing with the first gear (31 to 36), characterized in that the transmission mechanism unit further includes a transmission gear operation unit configured to operate the transmission gear unit, the drive shaft extends in the vehicle width direction through a portion between the transmission gear unit and the transmission gear operation unit, the drive shaft extends in the vehicle width direction through a portion between the transmission gear unit and plural shift rods (61, 62, 63, 64) of the transmission gear operation unit, and the plural shift rods (61, 62, 63, 64) are arranged to extend in the vehicle front-rear direction above the drive shaft. a transmission configured to perform gear-shifting and to output motive power input from a drive source; and a drive shaft that is coupled with the transmission and configured to transmit the motive power from the drive source, which is output from the transmission, to a drive wheel, wherein the transmission has a transmission mechanism unit that is arranged in a transmission casing such that an axis line extends in a vehicle front-rear direction and that performs gear-shifting for the motive power from the drive source, the drive shaft extends in a vehicle width direction on a central side of the transmission mechanism unit in the vehicle front-rear direction and is provided such that the drive shaft passes through the transmission casing, and the transmission mechanism unit includes an input shaft and an output shaft that are arranged on the same axis line, a counter shaft that is arranged in parallel with the input shaft and the output shaft, and a transmission gear unit that has plural gear trains (G1 to G6) each of which is formed with a first gear (31 to 36) provided to the input shaft or the output shaft and a second gear (41 to 46) provided to the counter shaft and meshing with the first gear (31 to 36), characterized in that the transmission mechanism unit further includes a transmission gear operation unit configured to operate the transmission gear unit, the drive shaft extends in the vehicle width direction through a portion between the transmission gear unit and the transmission gear operation unit, the drive shaft extends in the vehicle width direction through a portion between the transmission gear unit and plural shift rods (61, 62, 63, 64) of the transmission gear operation unit, and the plural shift rods (61, 62, 63, 64) are arranged to extend in the vehicle front-rear direction above the drive shaft.