PURPOSE: To carry out satisfactory work with excellent comfortableness to drive, by prohibiting turning operation in the opposite direction until a constant period attains from the time when a steering controlling means is changed over from turning to the left or right side to a stopped state.

CONSTITUTION: A limit switch (8a) on the left side is turned ON, values measured by a measuring means (D) and the set value are compared. If the measured values are larger than the set value, disconnecting actuation of the right transmission clutch is performed to carry out turn to the right side. The limit switch (8a) on the left side is changed over to the state of OFF to stop the intermittent disconnecting actuation of a transmission clutch (12b) on the right side. Thereby turning actuation in the opposite direction is prohibited until a time set from a point when state of turning actuation is changed over to the stopped state.