PURPOSE: To improve the accuracy of operation, by executing multiplication in the operation of a self-correlator by a circuit obtained by directly coupling an AD converter to a multiplication type DA converter.

CONSTITUTION: Digital signals (x) are successively outputted from a delay circuit 11 and multiplication with analog signals x(n) is executed by an operation processing circuit 20. These K products are inputted to a digital accumulation circuit 23 through a switch 24 and accumulation multiplied by a window function is executed. Consequently, the digital value of a self-correlation function Rn(k) is directly obtained from the accumulation circuit 23. The operation processing circuit 20 is obtained by directly coupling the multiplication type DA converter 21 with the AD converter 22. In the multiplication processing, the digital value of a previously digitized delay signal x(n-k) is supplied to the multiplication type DA converter 21 to find the product with the input signal x(n). The product x(n).x(n-k) is converted into a digital value by the AD converter 22.