A light position sensor comprising: light emitting means for emitting a light; an optical block (27) having a magneto-optical effect element (28) mounted thereon; a movable magnetic scale (13) disposed in opposed relationship to the magneto-optical effect element (28) and having a plurality of magnetized segments (29a to 29g) mounted thereon; first optical wave guiding means for guiding the light emitted from the light emitting means; polarizing means (26) connected at its one end to the first optical wave guiding means and at its the other end to the optical block (27), the polarizing means (26) being adapted to convert the light into a light having a single plane of polarization, the single plane of polarization being rotated to a predetermined angle by the magneto-optical effect element (28) in the presence of the magnetized segments (29a to 29g); polarization detecting means (30) connected to the optical block (28) in parallel relationship to the polarizing means (26) and adapted to pass therethrough a light wherein the single plane of polarization is rotated to the predetermined angle; second optical wave guiding means for guiding the light from the polarization detecting means (30) which is connected to the polarization detecting means (30); and opto-electric converting means (32; 69, 70) for converting the light guided by the second optical wave guiding means into an electric signal.