To provide a data transmission system capable of reducing skews between a reference signal and a phase modulation signal and accelerating data transmission.In a data transmission system having a transmitter for transmitting a reference signal and a phase modulation signal having a phase difference related to the value of input data from the reference signal and a receiver for obtaining data in accordance with the phase difference between the received reference signal and phase modulation signal, the transmitter transmits the reference signal and an adjustment signal having no phase difference before transmitting data and a control circuit variably controls delay values of variable delay devices so that the phase difference between the reference signal and adjustment signal received by the receiver decreases.