A data unit 1010 outputs an information signal to be transmitted in the form of optical packets, as well as wavelength information representing a wavelength of each optical packet. A first modulating signal processing unit 1051 receives an information signal, and inserts an electric signal (“dummy signal”) in any no-data period during which the information signal is absent, and outputs the resultant signal as a modulating signal. The dummy signal has the same amplitude as that of the information signal. A wavelength information processing unit 1021 outputs a wavelength controlling current corresponding to the wavelength information, and in any no-data period, outputs a wavelength controlling current corresponding to a wavelength which is different from the wavelengths represented by the wavelength information. A wavelength-tunable light source 1030 outputs light of a wavelength corresponding to the wavelength controlling current. An optical modulation unit 1040 modulates the light outputted from the wavelength-tunable light source 1030 with the modulating signal, and outputs the modulated signal as optical packets. As a result, an optical surge is prevented from occurring at the time of amplifying intermittently generated optical packets.