PURPOSE: To decrease the deterioration in the picture quality after transmission by making the generating timing of all television signals coincident with each other so that the horizontal synchronizing signal periods of all the television signals are overlapped with each other in an optical transmitter modulating the television signal.

CONSTITUTION: A horizontal synchronizing signal in a television signal of each channel is detected respectively by a synchronizing signal detection circuit 14. A delay time of a delay quantity setting circuit 16 is set respectively based on a detection timing obtained from each synchronizing signal detection circuit 14 so that the horizontal synchronizing signal periods of all television signals are overlapped with each other. A television signal of each channel delayed by the variable delay circuit 16 is amplitude-modulated by an amplitude modulator 17 into an RF signal and the signal is frequency-multiplexed by a mixer 18. After the obtained RF multiplex signal is amplified by an amplifier 10, the resulting signal drives a semiconductor laser 12. An output light from the semiconductor laser 12 is sent to a receiver side by using an optical fiber 13 as a medium.