PROBLEM TO BE SOLVED: To solve the problem that, in case of workings such as a planting work, transplanting work and the like by a usual mobile farm machine such as a rice planting machine and the like, it is difficult to carry out straight ahead workings. SOLUTION: An angular velocity sensor 63 for detecting the angular velocity of turning in lateral directions, an inclination sensor 64 for detecting lateral inclinations of a machine body, a speed sensor 65 for detecting the speed of the machine body and a power steering cylinder 44 for driving to steer front wheels 6 are installed on the machine body of a rice planting machine 4. Each of them is connected to a controller 46, detects deviations from the straight ahead direction and calculates compensation values and a steering control wheel is controlled and driven by the calculated values to go straight ahead.
