A passive optical network (PON) system which enables plural types of ONUs having different signal transmission speeds to be connected to one OLT. An optical line terminating apparatus (OLT) connected to plural types of ONUs having different signal transmission speeds through an optical distribution network includes an optical transmitter-receiver connected to the optical distribution network, a transmission/reception line interface connected to a wide area network, a downstream frame processing section for converting a packet received by the transmission/reception line interface from the wide area network into a downstream frame containing identification information on a destination ONU in a header, and a downstream transmission controller for modulating the downstream frame at a speed corresponding to a signal transmission speed of the destination ONU and outputting the modulated frame to an electrical/optical converter connected to the optical transmitter-receiver.