PURPOSE: To improve the efficiency of divers work or the like by towing magnetism detectors with towing cables to carry detection quickly in the deep sea bottom and separating a target setting machine when an abnormal magnetic substance is detected to output a detecting signal.

CONSTITUTION: A towboard 3 is floated on a sea area to be detected and a magnetism detector 1 and a target setting machine 4 are toward by a towing cable 2. If an abnormal magnetic substance exists in the sea bottom, the magnetism detector 1 detects the substance, the target setting machine 4 is separated by a discriminating signal outputted from a signal operation processing circuit and a sounding signal is outputted from a sounding body of the target setting machine 4. A controlling device 8 mounted on the towboard 3 records a position where the target setting machine 4 is set up. An abnormal magnetic substance recovering board searches the sound signal from the target setting machine 4 while referring the positional information for the target setting position received from the towboard 3, the abnormal magnetic substance can be easily recovered.