A diameter expansion drill bit used to expand the diameter of a part of a prepared hole bored in a frame by grinding includes: a cutting-blade portion that has a plurality of separate cutting-blade portions; a cutting-blade holding portion that slidably holds the plurality of separate cutting-blade portions in a radial direction; and a shank portion that supports the cutting-blade holding portion. The plurality of separate cutting-blade portions is arranged in a peripheral direction and arranged over a plurality of stages in a shaft direction, and the plurality of separate cutting-blade portions of the respective stages is caused to slide by a centrifugal force resulting from rotation so as to spread to an outside in the radial direction with respect to the cutting-blade holding portion.