PURPOSE: To realize equal ability in submarine detection in the entire worldwide area by providing one light source lamp and a couple of absorption cells and setting the center axis in an optical-axis direction at a specific angle to the horizontal direction.

CONSTITUTION: A light beam from the light source lamp 1 is made into parallel light by a lens 2 and the parallel light is supplied to the absorption cells 4 through a circular polarizing plates 3. Gaseous helium and a small amount of xenon gas are charged in the cells 4 and their output light beams are con verged on an adder 7 through photodetectors 5 and amplifiers 5; and the addition signal is processed by a signal processor 8 as specified and a specific signal is supplied to an R coil 9 to lock on this system at the optimum point of mag netic resonance caused by the cells 4, so that its lock frequency is sent out. The output light of this lamp 1 is set at 45° to the horizontal plane and at 120° to the horizontal plane direction respectively. Consequently, this system can be put in operation in the entire worldwide area.