PURPOSE: To enable indication of several pieces of target detection information on one display section by synthesizing information of target detector sections at several points through coordinate conversion.

CONSTITUTION: In a narrow sea area, monitoring is done from sites 1 and 2 each equipped with a target detector section adapted to detect a target transmitting an electromagnetic wave. Information at the site 1 is as seen in the Fig. 1 while information at the site 2 in the Fig. 2. When the screens 1 and 2 are synthesized, a coordinate conversion is done so that the centers of the Fig. 1 and 2 correspond to (a) and (b) respectively on the Fig. 3, which enables the indication of the overall monitoring area on one display section. At this point, the display color is changed depending on the target detected with a plurality of target detector sections and that detected with one target detector section alone to facilitate the identification of the target.