PURPOSE: To calculate a sample value at an arbitrary time from sample data sampled at a time interval (h) within a short operation time, by a method wherein it is checked whether one of multipliers is brought to a value near to 0 beforehand and, in the case near to 9, no multiplying operation is performed and a multiplying result is set to 0 uniquely to advance operation.

CONSTITUTION: A sample x(t) changing as the function of a time (t) is cyclic at a time cycle TP and sampled at even (N) points separated by a time (h) at an equal interval, that is, at t=h, 2h...Nh. From these sampled values, x(t) is calculated with respect to the arbitrary time (t) of a time (0, TP) section according to formulae I, II. Before the execution of the multiplication of the coefficient Aq and cos and that of the coefficient Bq and sin in the formula I, it is judged whether the coefficients Aq, Bq are near to 0 and, in a case near to 0, a result is set to 0 without performing multiplication to execute the operation of the formula I.