A drive power transmission device includes: a first shaft for receiving drive power transmitted from a drive source; a second shaft having a coupling shaft portion at a front end thereof; a coupling housing comprising a cover body having a front portion operatively coupled to a rear end of the first shaft and a rear portion including a central bearing portion in which a distal end of the coupling shaft portion is rotatably supported, and a tubular member having a front open end in which an outer circumference of the cover body is fitted and a rear boss end having a central through hole in which a proximal portion of the coupling shaft portion is rotatably supported, the cover body, the boss end, the coupling shaft portion, and the tubular member jointly defining a cylindrical fluid chamber filled with a fluid; a viscous shear coupling mechanism disposed in the fluid chamber for transmitting the drive power from the first shaft through the fluid to the second shaft; and a centrifugal clutch mechanism disposed between the coupling housing and the second shaft for transmitting the drive power from the first shaft to the second shaft.