An attachment pin assembly includes a pin body to be inserted into first through-holes of a drilling tool and a second through-hole of an adaptor, the first and second through-holes being arranged eccentrically with respect to each other; a bush portion provided on one end side of the pin body; a detachment prevention member of the bush portion screwed to one end of the pin body; and a stopper portion provided on the other end side of the pin body and configured such that a side surface portion arranged on the outside of the pin body in a radial direction thereof in the first through-hole is locked on an inner wall surface of the tooth forming the first through-hole to stop rotation of the pin body.