PURPOSE: To easily change operation characteristics by providing the title system with a factor rewriting means having a factor for previously determining plural operation characteristics and writing a factor selected based on a factor selecting signal generated from a digital filter in a rewritable area of the digital filter.

CONSTITUTION: A prescribed bit pattern is inputted from a prescribed input terminal of the digital filter 1 to an output circuit 12 in the digital filter 1 as a selection signal 59 through a matrix circuit 9. The output circuit 12 analyzes the selection signal 59 and outputs a multiplication factor selecting signal S12 to a microcomputer 8 as an analyzed result. The microcomputer 8 receiving the signal S12 stores a multiplication factor selected on the basis of the signal S12 in a register 4 of the digital filter 1 to rewrite the factor. Consequently, the system can be adapted to the characteristic change of the digital filter 1 without changing the filter 1 itself at all as hardware.