A position coordinate difference calculation section 5 calculates a position coordinate difference between a position coordinate of an output digital signal and a position coordinate of an input digital signal close to it. AN FIR coefficient memory 9 stores FIR coefficients of an FIR-LPF having such a characteristic as to cut off frequency components equal to or higher than ½ of an output sampling rate. When the position coordinate difference is input, the FIR coefficient memory outputs FIR coefficients corresponding to position coordinate differences between position coordinates of a certain number of input digital signals existing in the vicinity of the position coordinate of the output digital signal and the position coordinate of the output digital signal. AN FIR computation unit 3 performs FIR-LPF interpolation computation by using a certain number of the input digital signals and the FIR coefficients and obtains the output digital signal.