In a signal processing apparatus adapted to process a signal transmitted via a transmission path, distortion of a waveform of a signal value of a specific symbol is predicted on the basis of a characteristic of distortion depending on values of symbols transmitted before the specific symbol, and the distortion is removed from the waveform of the received signal thereby producing a distortion-removed waveform. A comparison value is calculated for each allowable value of the specific symbol by subtracting a predicted signal value of the specific value from the distortion-removed waveform. A symbol value corresponding to the smallest comparison value is determined as the value of the specific symbol. An error suspicion level value indicating the degree of suspicion of being incorrect is calculated for each of the predetermined number of symbols, and already determined values of symbols are corrected in accordance with the error suspicion level values.