A position detecting device comprises a pattern member having conductive patterns arranged in plural tracks; a slidable member having plural sliding pieces capable of sliding on the conductive patterns, wherein each of the sliding pieces assumes either a conductive state in contact with one of the conductive patterns or a non-conductive state not in contact with the conductive pattern; a signal processing device for detecting sets of the conductive patterns and the sliding pieces in conductive state, by transmitting electrical signals through the conductive patterns and the sliding pieces; and a discriminating device for identifying the relative position between the conductive patterns and the sliding pieces when the the conductive patterns and the sliding pieces in conductive state are detected at least a predetermined number of times or over at least a predetermined period of time.