Oxygen has an extremely strong paramagetism. When oxygen exists in non-uniform magnetic fields, the oxygen is attracted to the stronger magnetic field. In the detection, the glass dumbbell is horizontally hung on the platinum wire with the mirror. When oxygen passes through the detection section, a force works to extrude the dumbbell through the action of the above-mentioned magnetic fields. When dumbbell rotates, the position of reflected light that reached the photocell ( the light receptive section ) is changed. A counter torque is applied by feedback system to the dumbbell so that the changing dumbbell position can be returned to its original position. This counter torque is linear as compared with an oxygen concentration, and it is output as an oxygen concentration.