A self-calibrating key sensor system is constituted by an optical modulator, optical position transducers, a data accumulator and a data analyzer, and the optical modulator has a gray scale section to produce a key position signal varied together with the keystroke and an absolute data section to produce a calibration signal representative of a strictly adjusted distance between transparent areas of the absolute data section; the data accumulator accumulates discrete values of the calibration signal and discrete values of the key position signal together with the sampling time, and the data analyzer determines the gradient of the key position signal on the basis of peaks of the calibration signal produced at both ends of the strictly adjusted distance so as to estimate the true distance between the rest and end positions.