An R-wave of a heartbeat signal accompanied by periodic fluctuation is detected to obtain the average value of an RR interval. Further, a waveform of a heart sound signal, which periodically fluctuates in synchronization with the heartbeat signal, is forcibly repositioned at an interval equivalent to the average value of the RR interval. After performing reposition, noise is removed by using orthogonal transformation and orthogonal inverse transformation, so that the position of obtained waveform is restored to its original position.
