Diameter expansion drill bit

A diameter expansion drill bit inserted for use in a prepared hole bored in a frame and used to expand a diameter of a part of the prepared hole by grinding, the diameter expansion drill bit comprising:

• a cutting-blade portion that has a plurality of separate cutting-blade portions used to grind the part of the prepared hole;
• 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, wherein
• the plurality of separate cutting-blade portions is arranged in a peripheral direction and arranged over a plurality of stages in a shaft direction,
• 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,
• the plurality of separate cutting-blade portions of the respective stages has the same movement stroke in the radial direction, and
• the cutting-blade holding portion is formed into a shape spreading toward a tip end thereof.