Wavelet transformation was applied as a noise elimination method of an amperometric algal biosensor. The drift of the baseline current was clearly removed by using the wavelet transformation. The S/N ratio, calculated by the power spectrum density, is about 3-times larger than that calculated by the current response. The response to a herbicide, atrazine, calculated from the power spectrum density in high-noise circumstance, was the same as that calculated from the current response in a low-noise circumstance.