An object of the present invention is to provide a data transfer system capable of maintaining synchronization of the data during the operation of apparatus by performing the periodical adjustment for a DLL circuit without using a method of 8B10B. In a transmitting apparatus, a control portion switches a selector to start a readjustment for the DLL circuit in a receiving apparatus, if a counter indicates a readjustment interval time. In performing there adjustment for the DLL circuit, a pattern generator sends a pattern making a signal waveform a stationary wave through the selector for a certain time, and if the signal is made stationary, a readjustment pattern is sent to the DLL circuit. The DLL circuit resets the sampling timing, based on this readjustment pattern. The selector switches the normal data A and the readjustment pattern to be sent to the DLL circuit.