In a correlation arithmetic system adapted to detect a relative difference between two functions, an operation is simplified. This makes it possible to perform the operation with a small scale of hardware and also with great accuracy. There is adopted an operation g*h instead of the "product" in the correlation arithmetic operation. There is disclosed an arithmetic unit in which two numeral values a and b are inputted, and the two numerical values a and b are subjected to a predetermined operation process, so that a numerical value c representative of an operation result is derived. The arithmetic unit has an absolute value operation unit for evaluating an absolute value .vertline.c.vertline. of the numerical value c, and a sign operation unit for evaluating a sign "sing (c)" of the numerical value c.