PROBLEM TO BE SOLVED: To provide a four-wheel-drive running system that absorbs a speed difference between front wheels and rear wheels during low-speed running and turning, reduces a turning radius to the greatest possible extent, and is optimal for a work vehicle such as a speed sprayer.SOLUTION: A planet gear mechanism 8 is disposed on the side of a front end 7f, an intermediate part 7m, or a rear end 7r of a rear drive shaft 7, is coupled so that it can transmit a rotational output of an engine 3 to a sun gear 8s (or planet gear 8a) of the planet gear mechanism 8, and is coupled so that it can transmit a rotational output of the planet gear 8a (or sun gear 8s) of the planet gear mechanism 8 to rear wheels 6p and 6q. The periphery 8if of an internal gear 8i of the planet gear mechanism 8 is threaded to be formed as a worm wheel 8w. A speed variation mechanism Xv includes a worm gear 9 that is meshed with the worm wheel 8w and a rotation drive unit 10 that rotates the worm gear 9, and a drive control unit 12 drives and controls the rotation drive unit 10 according to a steering angle Qs of a steering wheel 11.【課題】 低速走行旋回時に前輪と後輪の速度差を吸収し、旋回半径を可及的に小径化する特にスピードスプレーヤ等の作業車両に最適な四輪駆動走行装置を提供する。【解決手段】 リアドライブシャフト部７の、前端部７ｆ側，中間部７ｍ又は後端部７ｒ側に遊星歯車機構８を介在させ、エンジン部３の回転出力を、遊星歯車機構８の太陽歯車８ｓ（又は遊星歯車８ａ…）に伝達可能に結合し、かつ遊星歯車機構８の遊星歯車８ａ…（又は太陽歯車８ｓ）の回転出力を後輪６ｐ，６ｑ側へ伝達可能に結合するとともに、遊星歯車機構８における内歯車８ｉの外周面８ｉｆに歯を設けることによりウォームホイール８ｗとして構成し、このウォームホイール８ｗに噛合するウォームギア９及びこのウォームギア９を回転させる回転駆動部１０を有する速度可変機構Ｘｖと、ハンドル１１の操舵角Ｑｓに対応して回転駆動部１０を駆動制御する駆動制御部１２とを備える。【選択図】 図１
