PURPOSE: To maintain a submerged body at a certain depth without providing a power supply or the like by driving a sea water pump by means of a screw to take in sea water and by feeding it into a cylinder mechanism adapted for actuating a horizontal rudder via an operation valve which can be changed over according to the depth.

CONSTITUTION: A submerged towed body 3 which is towed via a rope by a mother ship has a screw 11 at the rear. A sea water pump 8 is driven by revolution of the screw to feed sea water by a change-over valve 9 into an operation cylinder 7 of a horizontal rudder 4 which is adapted for varying the depth of the towed body, whereby the horizontal rudder 4 may be actuated. The change-over valve 9 is operated by a differential pressure mechanism 10 which detects the preset depth and an actual depth via links 21 and 47. The preset depth is applied to a small diameter piston 41 via a spring 56 in term of a torque by turning of a handle 67 while the pressure of sea water coming from an inlet hole 5 of the towed body 3 is applied to a large diameter piston 42. The mechanism 10 detects the differential pressure between those pistons, namely detects a difference between the preset depth and the actual depth.