An influence on a sensor output is suppressed to a minimum degree even in the case when there is a ripple in a light source, thereby improving the measurement accuracy. In an intensity modulation type optical sensor, light from a light source 1 is guided to a sensor head 6, the intensity of the light is modulated on the basis of an alternating current (AC) measured object (for example, a current i) changing with time, the modulated light is received in a light receiving element 81 and converted into an electrical signal, and a normalized received signal Xs indicating a degree of modulation is acquired from a ratio of an AC component As and a DC component Ds of the electrical signal to thereby acquiring a value of the AC measured object. Further in this invention, a reference signal Vr is acquired by separating and receiving part of light incident on the sensor head 6, a normalized reference signal Xr is acquired from a ratio of an AC component Ar and a DC component Dr of the reference signal, and the normalized reference signal Xr is subtracted from the normalized received signal Xs, thereby reducing noises and improving the accuracy.