1. A decoder comprising: setting circuitry configured to set, as a decoded region, a region of a decoding target image that is adjacent to a target block of the decoding target image, where the target block is a target to be decoded and the decoded region is decoded earlier than the target block, and to set, as a reference region, a region of a reference image that has a first positional relationship with a reference block of the reference image that is the same as a second positional relationship of the decoded region of the decoding target image with the target block of the decoding target image; calculating circuitry configured to calculate a filter coefficient of a filter that converts pixel values of the reference region set by the setting circuitry into pixel values of the decoded region set by the setting circuitry; interpolating circuitry configured to interpolate pixel values of the reference block by using an adjustment filter having the filter coefficient calculated by the calculating circuitry; and generating circuitry configured to generate a prediction image corresponding to the decoding target image by motion-compensating the reference block having the pixel values interpolated by the interpolating circuitry for the target block, wherein an edge of the reference region abuts an edge with the reference block in the reference image and an edge of the decoded region abuts an edge of the target block in the decoding target image.