It is an object of the present invention to provide a DSB-SC system capable of suppressing a third order component. The DSB-SC modulation having high extinction ratio can be realized by adjusting the first order component, which is generated by applying a modulation signal (3f.sub.m), and the third order component, which is generated by applying a basic signal (f.sub.m), to have reversed phase and the same intensity level, and then by applying the first order component to the third order component, these two components cancel each other.