PURPOSE: To improve the safety of the working by providing approach sensors to the left side corner and the right side corner of a counterweight provided at the rear side of an upper side turning body, and inputting a signal from the approach sensor to a controller when the corner part approaches an obstacle, so as to output an alarm generating signal.

CONSTITUTION: When an obstacle detecting signal is input to a controller 9 from at least one approach sensor of approach sensors 5, 6, and 7, the controller decides depending on the signal, and outputs an alarm generating instruction signal to an alarm generating device 10. In this case, when an interval and distance representation instruction signal is output from the controller 9 to the voice massage data holder 11 of the alarm generating device 10, an alarm is generated in a human voice from a voice generator 12 through the voice massage data holder 11.