Wavelet transformation was applied as an elimination method of influence by an interfering substance on an amperometric biosensor. The current responses of a bi-enzyme type lactose biosensor in a solution containing both lactose and ascorbic acid were analyzed by wavelet transformation. The power spectrum density for ascorbic acid was detected selectively at around 0.125 Hz. The current component due to ascorbic acid was eliminated at 98% from the current response of the biosensor by wavelet transformation.