PURPOSE: To detect automatically a liquid leaked from a valve, etc., and to reduce a burden on a plant monitoring person, by providing an image processor on a monitor system, and deciding a pool by processing a variable density image which is caught by an ITV camera.

CONSTITUTION: A general purpose image processor 8 is provided additionally on a monitor system consisting of an ITV camera 3, an illumination use power source 4 and a monitor television 5. In such state, smoothing of a variable density image which is caught by the ITV camera 3 or smoothing which has held the edge is executed, and subsequently, an edge detection is executed to the image which is obtained by smoothing, by using eight kinds of differential operators. Next, the edge detected image is brought to a binary-coding processing by a prescribed threshold value, and thereafter, a variable density value of a boundary is extracted by executing a picture element expansion processing and a differential processing, and a pool is decided by detecting the existence of a sudden variation point. In such a way, a pool, etc. which have leaked from a pipe, a valve, etc. can be detected automatically.