A raw signal is delayed in a delay circuit (1) and then supplied to one of the input terminals of a selection circuit (8). The raw signal is also supplied to another delay circuit (2) to be delayed. Subsequently, data of this delayed signal is processed by spline interpolation in an interpolation circuit (4), and the interpolation values are supplied to the other of the input terminals of the selection circuit (8). Further, a timer (7) is operated according to an output signal from a noise detection circuit (6) which detects noise in the raw signal. When noise is detected, the selection circuit (8) switches to select the output from the interpolation circuit (4), thereby achieving interpolation of noisy sections.