PURPOSE: To simplify the signal processing and miniaturize the sensor structure, by setting a couple of receivers whose detecting zones are set so that one of them can detect a seedling near the machine body and the other, another seedling a little far from the former seedling by a certain distance.

CONSTITUTION: When the first receiver 8b detects a seedling W, the steering operates so that the machine body goes away from the seedling, while the body tries to get near to the seedling W, when the second receiver 8c detects the seedling W. While neither of them detect the seedlings, the steering operation is not done, under the judgment that the seedlings are suitably arranged to the distance D. Thus, the situation between the seedlings and the machine can be judged simply based on the signals from the second receiver and the structure of sensors can be miniaturized.