PURPOSE: To prevent a steering wheel from being operated inadvertently during rectilinear travel so as to secure straight line stability while improving the operability of the steering wheel at the time of turning in a paddy field working machine provided with power steering mechanism.

CONSTITUTION: Pressure sensors 18 are internally mounted in the rotational center side part of a steering wheel 7. When the inside held state of the steering wheel 7 is detected, power steering mechanism 13 is operated to change its assist power onto the strong side, and when the inside held state is not detected, the power steering mechanism 13 is operated to change its assist power onto the weak side.