A deblocking filter applies filtering to reduce blocking artifacts from an encoded stream in which image data has been encoded in individual blocks. A filter strength adjuster adjusts the filter strength according to block sizes of the predictive image data. For example, during image encoding, the filter strength is adjusted according to the block size of the predictive image data yielding the best encoding efficiency generated using filtered and decoded image data. Filtering may also be applied to remove blocking artifacts from decoded image data during image decoding, with the filter strength being adjusted according to the block sizes of the predictive image data. Decoded images with favorable image quality are obtained, even in the case of using extended macroblock sizes.