To provide an active object detector where workability is improved by allowing easy setting of a string of effective detection areas.

The active object detector includes a projector 1 for transmitting detection wave for object detection towards a plurality of detection areas A1-A5, and a photodetector 2 for receiving detection wave reflected by the object and generating a light receiving signal. The detection areas A1-A5 are arranged from a position close to the projector 1 toward a position far from it in a plurality of strings, and the photodetector 2 has a plurality of different light receiving elements 8A-8F for receiving light in each of the detection areas A1-A5. The active object detector further includes a detection reporting means 19 for generating light of different color or different sound in each of the detection areas A1-A5 when light receiving elements 8A-8F corresponding to respective strings of the detection areas A1-A5 output a light receiving signal.