PURPOSE: To enable an accurate start of automatic straight-advance control, by detecting respective operatable positions by both ground-contact and lift sensors and carrying out the straight-advance control by means of an automatic straight-advance switch corresponding to the detection.

CONSTITUTION: In case an automatic straight advance switch 6 is adjusted to ON in such a state that a sensor 2 for detecting approach of a working device 1 to the soil surface and locating thereof at the operatable attitude position and a lift sensor 4 for detecting lowering of the working device 1 and a car body 3 to the operatable attitude position may simultaneously show operatable attitude, an automatic straight-advance control can be carried out. Thereby, the automatic straight-advance control can be started under the accurate operatable attitude and maloperations can be prevented.