PURPOSE: To realize an asynchronous sampling frequency conversion system which relaxes necessary time resolution greatly by introducing a multirate signal processing method.

CONSTITUTION: Three zero points are interpolated during one sample period in the 1st discrete signal sequence of sampling frequency fS inputted to an input terminal 30 and this is regarded virtually as a fast discrete sequence of sampling frequency 4fS and inputted to a register 305. A signal obtained as the output of an adder 309 is the 1st fast discrete signal sequence of sampling frequency 4fS. A numeral interpolation part 330 generates the 2nd fast discrete signal sequence of sampling frequency 3fS' on the basis of said 1st fast discrete signal sequence. The 2nd obtained fast discrete signal sequence is passed through the low-pass digital filter composed of a register 315, multipliers 316, 317, and 318, and an adder 319 to obtain a smoothed output.