Abstract: An apparatus of this invention is directed to an information processing apparatus that continues detection of an instruction of an operator even if the reliability of instruction information detection has lowered due to a device fault, degradation of the environment, and the like.

Abstract: To supply a frequency control signal to a laser diode module for generating a laser beam susceptible to a variation in frequency or wavelength, a frequency stabilizing device comprises a birefringent body (21) for converting the variation to a variation in a polarization state of the laser beam to produce a polarization varied beam, which a polarization separating element (22) separates into first and second beam components having orthogonally polarized directions and first and second beam intensities. Optical detectors (26,27) detect the beam intensities to produce first and second detection signals. A subtracter (28) calculates a difference between the beam intensities. A control signal producing arangement (29) produces the frequency control signal in compliance with the difference. Preferably, the difference is normalized by a sum of the beam intensities. The birefringent body may be a uniaxial or a biaxial body. The uniaxial body should have an optic axis which forms an angle of 45.degree.