PURPOSE: To monitor an equipment to be monitored without disturbing the operating state of the equipment by allowing a display lamp or a LED fitted to a unit to be monitored to attract a photosensor and extracting the information of the display lamp.

CONSTITUTION: A signal obtained from an attraction type photodetector 1 including a permanent magnet 11 and the photosensor 13 is converted into a TTL level by a level converter 2 to light up a current state displaying LED 3 and held by a latch circuit 4 to simultaneously light up a latch displaying LED 5. The output of the latch circuit 4 is inputted to a data collecting circuit 8 and then outputted to a host computer 9. In this case, an analog input has 16 channels and 8 channels out of them insulate I/O through an isolation amplifier 6, so that the operating state of the equipment to be monitored can be prevented from being disturbed. The signal converted by an A/D converter 7 is inputted to the circuit 8 and then outputted to the host computer 9.