PURPOSE: To achieve a higher safety with automatic stoppage of a construction machine by outputting second and third alarm signals in succession as a first alarm signal is outputted when a radio wave and an ultrasonic wave received overlap hourly.

CONSTITUTION: Ultrasonic wave transmitters 12a and 12b receive 34 transmitted radio waves Aa and Ab of a radio wave transmitter 10 while transmitting ultrasonic waves Ba and Bb. Then, ultrasonic wave receivers 15c-15e output received signals Cc-Ce to an alarm signal output circuit 39 while receiving 34 the radio waves AcAe and receive 38 signals turned to ultrasonic waves Bc-Be with a time delay and an amplitude attenuation due to a distance from the ultrasonic waves Ba-Bb and outputs signals Dc-De to the circuit 39 while receiving the signals. Then, when the signals Cc-Ce and the signals Dc-De are both inputted, the circuit 39 outputs alarm signals Ec-Ee to serve as drive signal for an alarm or control signal for other devices while second alarm signals Fc-Fe are transmitted 41. Alarm signal receivers 17a and 17b receive 43 and outputs a third alarm signal through a second alarm signal output circuit 44 to serve as control signal for the alarm or other devices.