The detection performance of sonar is shown with Figure of Merit (FOM). The relation between detection distance to the target and FOM was not able to be compared directly in real of the sea field so far. The reason why is not to be able to measure the noise level and the array gain in the sea at the same time when the detection is examine because they always change. The measuring method of FOM is shown here at the same time when the target detection is examined in a real of sea field.