PURPOSE: To obtain an economical, small-sized monitor device which has high reliability, maintainability, and operativity by multiplexing and processing detection signals from plural radiation detectors.

CONSTITUTION: The plural radiation detectors 1 are connected to a signal processor 3, and detection signals are converted into digital signal, multiplexed, and supplied to a monitor unit 5, which transmits data on a set voltage, etc., to the detectors 1 through the processor 3. The arithmetic microcomputer 15 of the unit 5 receives and stores successively plural radiation detection signals which are supplied from the detectors 1 through the processor 3, and processes the detection signals variously to obtain specific arithmetic results of the calculation of a radiation level, etc. Further, a microcomputer 17 for display control receives various data supplied from the arithmetic microcomputer 15 and processes them, and a display unit is controlled to display an image plane.