PURPOSE: To reduce the scale of a data accumulator and also to shorten the delay time of data communication by using the data block start information, the data block end information and the data output timing information to control the data accumulator.

CONSTITUTION: A data output timing generator 5 informs the data output timing information to a data accumulator 1 based on the known finite data block length as well as the synchronous differential information received from a synchronous information differential detector 4. In other words, the generator 5 informs the data output timing information to the accumulator 1 immediately after receiving the data block start information from a data block start detector 2 if the data communication speed of an S2 synchronizing system is slower than that of an S1 synchronizing system. Then the initialization is performed based on the data block end information issued from a data block end detector 3 so as to produce the next data output timing information. The accumulator 1 starts output of the stored input data by means of the S2 synchronizing information at and after a time point when the data output timing information is received from the generator 5.