PURPOSE: To detect the entrance of one or a plurality of operators into the dangerous areas of each service car positively in a job site, in which a plurality of the service cars and the operators working on the outside of the service cars work.

CONSTITUTION: An ultrasonic sender 12 and a radio transmitter 13 are installed to an operator 2, and an ultrasonic receiver 10 and a radio receiver 14 are mounted to a service car 1 composed of a hydraulic shovel 1A, an excavator 1B, etc. A reference radio signal is transmitted by the radio transmitter 13 from the operator 2 side while an ultrasonic pulse signal is transmitted by the ultrasonic sender 12. A time when the ultrasonic signal is received after the reference radio signal is received on the service car 1 side is measured by a time measuring instrument 32. The time is compared with a reference time set to a reference setter 34 by a comparator 33, and whether or the operator 2 enters into a dangerous area in either service car 1 is.