An encoder includes processing circuitry; and a memory coupled to the processing circuitry. Using the memory, the processing circuitry is configured to: change values of pixels in a first block and a second block to filter the boundary between the first block and the second block such that change amounts of the respective values are smaller than respective thresholds, the pixels being arranged along a line across the boundary; and encode a third block. The pixels in the first block include a first pixel located at a first position, and the pixels in the second block include a second pixel located at a second position corresponding to the first position with respect to the boundary. The thresholds include a first threshold and a second threshold corresponding to the first pixel and the second pixel, respectively. The first threshold is different from the second threshold.