PURPOSE: To reduce remarkably capacity of a memory in keeping its accuracy, by executing a prescribed arithmetic for a data read out from the memory in accordance with an input data.

CONSTITUTION: In a function generator for generating a command of servo-control, a function is stored in advance in a memory 4, and in accordance with a value (x) of an input 1, a function f(x) is read out as an output 5 from a function memory 4. On the other hand, the input 1 is supplied to an arithmetic device 6, and a function g(x) is obtained as its output 7. The output 7 of the device 6 and the output 5 of the memory 4 are applied to an arithmetic device 8, a prescribed arithmetic is executed, and a function h[g(x), f(x)] is generated as an output 9. In this way, a necessary memory capacity can be reduced without degrading the accuracy.