In the detection of underwater target, there exists an optimal depth for the sonar systems where the detection probability is maximized. In contrast, for the targets, there is a target depth where the detection probability by the sonar system is minimized. In this paper, we address this question from the view point of the target, that is to find the target depth where the detection probability of the target by sonar systems is minimized. The detection probability of the target is dependent on the depths of target and sonar, the submarine topography, the sound speed profile and the generated frequency of the target. In this paper, the detection probability of the target is calculated by the depth for the slope direction of the submarine slope based on the information about the sound speed profile and the submarine topography in the ocean.