PURPOSE: To miniaturize a device, to unnecessitate an adjustment and to reduce the unstable elements such as the secular change of a characteristic by constituting the device by all the logic circuits without using analog parts such as a low-pass filter and an oscillator, in a data rate averaging device absorbing the time fluctuation of input data.

CONSTITUTION: Input data is inputted in a FIFO memory 12, a packet terminal circuit 11 outputs data length information showing the length of data to be transmitted within time T, and a stuff period calculation circuit 13 inputs data length information and outputs stuff insertion period information. A stuff pulse generation circuit 14 outputs a stuff timing signal from the stuff insertion cycle information and a reading control circuit 15 controls the output of the FIFO memory 12 according to the stuff timing signal.