A signal transmission system has a modulation signal converter 1 that generates a modulation signal using a Manchester code with a duty ratio of 50% in accordance with transmission data; a clock generator 6 that generates a clock with the amount of delay with respect to the rising or falling edge of the modulation signal; and a data detector 5 that generates received data by sampling the modulation signal in synchronization with the clock. Since the modulation signal converter 1 generates the modulation signal by combining the Manchester code with the duty ratio of 50%, its duty ratio is always 50% independently of the transmission data, thereby preventing the DC offset of the modulation signal on the receiving side. Accordingly, it offers an advantage of achieving good communication quality with a simple circuit configuration without producing the DC offset in the modulation signal on the receiving side.