In the acoustic equipment of Sonar Systems used in transponders or moving bodies, a projector is mounted on one end of a cylindrical tube. From this configuration, the reflective wave from the tube end is induced, and the directivity pattem of the projector is degenerated. This effect can be minimized by formulating a correctly shaped reflecter (;baffle) and inserting it between the projector and the tube. However, since there are few methods for calculating the acoustic field of such projector, how to design the reflecter shape to obtain a desireble directivity pattern is not completely clear. Hence, in this paper, we suggest an useful method for calculating the acoustic field so that we can provide a means of determining the best shape for the conical reflecter.