PURPOSE: To reduce the deterioration of S/N of a signal caused by frequency conversion by converting a sampling frequency between digital signals without interposing an analog signal and to compress the circuit scale of a system by changing the sampling frequency without using an A/D converter, a D/A converter, and an over-sampling circuit, etc.

CONSTITUTION: A timer 119 issues an interruption signal which informs the input of data to an I/O 118 to an instruction decoder 17 at every input of sampling clock of an input digital signal. The instruction decoder 17 outputs an output digital signal based on the interpolation result of input digital data corresponding to the interruption signal of an output request issued to the instruction decoder 17 at every input of an output sampling clock from a timer 223.