PURPOSE: To prevent error in levee detection, by using a pair of sensors having detection areas staggered in lateral direction and outputting a control signal exclusively when a prescribed number of continuous detection signals are outputted from both sensors in a prescribed number of successive sampling operations.

CONSTITUTION: Transmitter-receiver units 8, 8 for controlling sensors 7, 7 perform detection operations at prescribed intervals by a signal transmitted from a sampling signal generator 9. When the distance from the detected levee B is shorter than a prescribed level, a detection signal is transmitted to a levee- discrimination means 10 having an AND-condition discrimination part 10a and a counter 10b. The input signals are counted by the counter 10b exclusively when the output signal from the discrimination part 10a is synchronized with the sampling signal. When the number of counts reaches a level preset by a setter 11, a control signal is outputted.