GPS has become an indispensable tool in fishery by fishing boat recently. A positioning accuracy (horizontal deviation: 2DRMS) has been 15.9m by SGPS and 4.5〜6m by DGPS in case of HDOP is 1.5 after cancellation of selective availability (SA). The positioning accuracy improved 10 times as compared with after GPS started. Although GPS is such a convenient tool, the main usages of fishery are safety navigation and search of fishing point. In this paper show that a possibility of the application to the monitoring of a movement of fishing gear and the management fishery by DGPS.