To reduce a bit error rate by generating a continuous wave output signal operable on the prescribed light carrier wavelength, changing the intensity of the signal, and reducing the amplitude of residual amplitude modulation in response to the change of the intensity.

Regarding a transmission system with a wavelength dither transmitter 10, a receiver 17 and an optical transmission fiber 15 for connecting the transmitter 10 and the receiver 17 to each other, the laser 11 of the transmitter 10 biases a laser beam for the purpose of continuous wave operation, and outputs a continuous wave signal to operate on the preset light carrier wavelength. Also, a dither source 12 dithers the light carrier wavelength of the laser 11 at a periodical speed Fd equal to or above C/2nLeff, where C stands for a luminous flux in vacuum. (n) for an effective refraction factor and Leff, stands for an effective optical fiber transmission distance. In addition, an external modulator 14 changes the intensity of an output signal from the laser 11, and a high saturation amplifier 15 having a low cutoff frequency fc (fc≥fd) equal to or above C/2nLeff, reduces the amplitude of residual amplitude modulation on a signal from the external modulator 14, in response to the single.