A drive power transmission device includes an input carrier having a front portion for receiving drive power from a drive source and a tubular rear portion, a planetary gear mechanism having a sun gear disposed coaxially in the tubular portion of the input carrier, a plurality of planet gears meshing with the sun gear and rotatably supported on a periphery of the tubular portion, and a ring gear mounted concentrically on the tubular portion and meshing with the planet gears, a first output shaft coupled to and supporting the sun gear, an intermediate tubular member connectable to the first output shaft and a rear end of the tubular rear portion of the input carrier and disposed concentrically with the first output shaft, an outer tubular member connectable to the ring gear and disposed concentrically with the intermediate tubular member, a viscous shear coupling mechanism interconnecting the outer tubular member and the intermediate tubular member, and a second output shaft coupled to and supporting the outer tubular member. The viscous shear coupling mechanism comprises two groups of mutually interleaves plates disposed in a fluid chamber defined between the intermediate tubular member and the outer tubular member and filled with a fluid and fixedly fitted over the intermediate tubular member and in the outer tubular member.