PURPOSE: To obtain an interference sensor with stable characteristics by providing a light branch coupler wherein a light phase difference between transmission light and branch light has been adjusted to a predetermined value as an optical element of an optical system.
CONSTITUTION: A signal processing circuit 50 receives an interference signal from a light detector 40 which varies sine-functionally with respect to a phase difference ϕs and outputs linear signal voltage with respect to the phase difference ϕs by inverse sine functional calculation. When a light amount from a laser light source changes, an amount of received light of the light detector 40 immediately varies. On the other hand, there may be a fluctuation in the light amount in an interference sensor employing an optical fiber due to a temperature of coupling efficiency between the light source and the optical fiber or due to a change with time, etc. Therefore, such a problem occurs that a proportional coefficient of output voltage of the signal processing circuit 50 and the input phase difference ϕs, in other words a voltage scale, varies due to an amount of incident light of the light detector 40. Thus a second light branch coupler 60 and a second light detector 41 are additionally provided. Therefore, as a result, output light from a second terminal 62 of the second light branch coupler is also maintained at a constant light amount.