To provide a digital filter where filter processing time is fast and pointer management is easy and which has the number of finite input data for a general purpose microcomputer.

A delaying device 1 has the arrangement of delay elements of the number totalizing the number N of data of input data stream and the number (p) of delays, stores zero in p delay elements from a 1st end side of the arrangement and successively stores N input data of the data series in N delay elements from the (p+1)-th delay element counted from the 1st end side toward the a 2nd end side in the order of the series when filter processing is initialized. A 1st pointer device PM1 designates a delay element in which the data of a 1st term of a product-sum operation is stored and moves the designation position in every time that corresponds to the delay time of the delaying device 1. A 2nd pointer device PM2 designates a delay element in which data multiplying a tap coefficient is stored.