PURPOSE: To prevent the occurrence of trouble by monitoring an operation item where the operation state of the whole device appears intensively.

CONSTITUTION: A target detecting means 1 outputs a reflected receive signal from a ship to be monitored and a reflected receive signal from a fixed target. Its output signal is applied to an extracting means 3 which constitutes a decision means 2, and only the signal from the fixed target is extracted. The output signal of the extracting means 3 is applied to an amplitude detecting means 4 and a detection probability detecting means 5 to detect an amplitude and detecttion probability, which are sent out to a comparative decision means 6. The comparative decision means 6 compares the reception amplitude and detection probability of the fixed target when the target detecting device is in normal operation with the reception amplitude and detection probability which are obtained currently, thereby deciding on the operation state of the device.