An optical detector for detecting at least three optical beams reflected by an optical information recording medium and outputting an electric signal, the optical detector having three light receiving areas each of which receives a respective one of the three optical beams and each of which has four light receiving surfaces, twelve signal lines to transmit an electric signal obtained on respective ones of the four light receiving surfaces of the three light receiving areas, selected ones of the twelve signal lines connected to the four light receiving surfaces of different ones of the three light receiving areas being joined together within the optical detector, and less than nine signal output lines formed by joining the selected ones of the twelve signal lines extend from the optical detector to a signal processing circuit.