methods of the controller 43 a the through hole 331 is formed proximal to the front end of the light isolator 33 when the through hole 331 is formed proximal to the front end of the light isolator 33 and the reflector 10 is not yet completely screwed to the sensor 20 as shown in fig2 the light emitter 41 is isolated from the light sensor 42 by the light isolator 33 when the reflector 10 is completely screwed to the sensor 20 and pushes the light isolator 33 to a predetermined position the through hole 331 aligns with the light emitter 41 and the light sensor 42 and a part of a light beam emitted from the light emitter 41 can directly impinge on the light sensor 42 through the through hole 331 as shown in fig3 with reference to fig4 when the reflector 10 is fixed in a wall 51 of a container or a tank and is not immersed in liquid the light beam output from the light emitter 41 is totally reflected by the reflection head 11 to the light sensor 42 with reference to fig4 a on the other hand when the reflector 10 is immersed.