The transponder includes a 64B/66B code synchronization bit determination circuit 1511a for notifying a PLL selection control circuit of a control signal which instructs on selection of a variable oscillator or a variable oscillator in response to a case where an electric signal sent from an optical signal reception unit 111 includes a 64B/66B code synchronization bit, an A1A2 byte determination circuit 1511b for notifying the PLL selection control circuit of a control signal which instructs on selection of a variable oscillator or a variable oscillator in response to a case where an electric signal sent from the optical signal reception unit 111 includes an A1A2 byte, and the PLL selection control circuit 1512 for controlling a PLL selector 16 and a PLL selector 17 based on a control signal.