To suppress the amount of data in a buffer within a certain range at all times by measuring the amount of data which are currently stored, and comparing it with a previous value and deciding whether the data increase or decrease.

Input data Di is inputted to a counter 73, the quantity of output data Do is inputted to a terminal from a reversible counter 73 and subtracted, and Di-Do=DA is outputted as the current amount of stored data in the buffer 71. The data amount DA is inputted to a latch 74, whose latch data DDA is sent to a comparison part 75. The comparison part 75 has inputted the current data amount DA measured by the counter 73, so a comparison part 75 can obtain whether the amount of data stored in the buffer 71 increases or decreases and sends that as a comparison result to a decision part 78. The decision part 78 decides whether or not the comparison result CM matches information data ST set in an information data setting part 77 and outputs information CS to a CPU when their matching is decided.