To provide a power transmission device having simple construction for setting and holding a neutral condition while smoothly changing over forward/reverse travel.

The power transmission device comprises a positive-rotation first driving pulley 51A connected to a crank shaft 27 of an engine 12 and a reverse-rotation second driving pulley 51B, a first input shaft 61 connected to a driving shaft 121, a first pulley 64 mounted on the first input shaft 61, a V-belt 52 bridged over between the first driving pulley 51A and the first pulley 64, a second input shaft 62 connected to the driving shaft 121, a second pulley 66 mounted on the second input shaft 62, a V-belt 53 bridged over between the second driving pulley 51B and the second pulley 66, a turning mechanism 90 for tensing only one of the V-belts 52, 53 to change over the belt to transmit power, and a holding mechanism 70 for slackening both V-belts 52, 53 to hold the neutral condition of interrupting the transmission of power.