A first hydraulic pump P1 driven by power of an engine and a first hydraulic motor M1 are fluidly connected so as to form a closed fluid circuit. An output shaft 2 of first hydraulic motor M1 is drivingly connected with a sub-transmission 3. The output power from sub-transmission 3 drives main-driving wheels 5 and also drives a second hydraulic pump P2 fluidly connected with a second hydraulic motor M2 for driving sub-driving wheels 6. A clutch 7 is interposed between an output shaft 4 of sub-transmission 3 and an input shaft 9 of second hydraulic pump P2 or on the output side of second hydraulic motor M2. Second hydraulic pump P2 or second hydraulic motor M2 has variable displacement, so that second hydraulic motor M2 is drivingly accelerated in proportion to the degree of turning operation of a steering operating tool 31. In case that a plurality of front reel mowers 8FL, 8FM and 8FR are disposed in front of the vehicle body, a pair of decelerator casings 29 are disposed on both lateral sides of transmission casing 1 for driving main-driving wheels 5. Decelerator casings 29 project horizontally forward and support main-driving wheels 5 respectively at their front portions. Middle front reel mower 8FM is disposed between left and right decelerator casings 29.