An object of this invention is to efficiently attain noise removal. In order to achieve the object, the discrete wavelet transforms of an input image are computed to output wavelet coefficients of respective subbands (S301). Appropriate threshold values are respectively set for subbands HL, LH, and HH indicating high-frequency components (S302a, S302b, S302c). The wavelet coefficients of the subbands HL, LH, and HH then undergo threshold value processes using the set threshold values for the respective subbands (S303a, S303b, S303c). Pixels to be processed in a coefficient conversion process are determined based on the threshold value processing results of the respective subbands (S304). The wavelet coefficients of the respective subbands corresponding to the pixels to be processed determined in step S304 undergo coefficient conversion (S305), and the converted transformation coefficients undergo inverse discrete wavelet transformation, thus reconstructing and outputting a noise-removed image (S306).