To generate a signal in the slippage from a calculated position without regularly generating a previous announcement signal by generating a signal differed from the previous announcement signal on the basis of a signal conformed to a set angle.

A comparing means 15 compares the detected angle value of a horizontal angle encoder 4 from a counter 10 with the set angle and previous announcement angle from an announcement angle generating means 14, and when the both are conformed to each other, a conformation signal is outputted. The comparator 15 also judges whether the detected angle value is conformed with the previous announcement angle in the direction approaching the set angle, or in the direction leaving the set angle, and transmits the result to an alarm signal generating means. The alarm signal generating means generates distinguishable previous announcement signals with varied tones or rumbling intervals for a specified time on the basis of the direction of the conformation of the detected angle value with the previous announcement angle. Thus, a worker can confirm the regulating direction to a calculated position.