In a control apparatus, a "characteristic value calculation" block calculates a characteristic value (such as amplitude value, offset value, or waveform distortion for each phase of two-phase sinusoidal signals optically input from an encoder, or phase difference between the two phase signals) for each of the two-phase sinusoidal signals, and an "alarm detection" block checks the presence or absence of an excursion outside a predetermined allowable range and, if such an excursion is detected, produces an alarm indication or the like. Each time the "present characteristic value" is input, a "characteristic value comparison" block compares it with "previous characteristic values", and analyzes the result of the comparison. That is, the difference between the present value and each previous data is calculated, and the largest amount of variation (with plus or minus sign) is obtained; if this amount is larger than a predetermined value, a signal indicating an "imminent failure" is output. In an alternative method, the present value is compared with a characteristic value calculated from an encoder reference signal. Further, the characteristic value monitoring and comparison may be performed within the encoder. In this way, data useful for the prediction of encoder failure, etc. is provided.