To facilitate handling of an undercut drill by simplifying a structure of the drill.

The under cut drill comprises a hollow drill shaft 11 having a rear end 111 coupled to an engaging hole 221 of a rotary cylinder 22 and a long groove-like cutter blade containing space 110 axially formed at its distal end, a push bar 12 slidably inserted into the shaft 11 and having a rear end contacted with the distal end of a reciprocating piston rod 23, a large-diameter part 121a and a small diameter part 122 of the distal end formed thereat, and a cutter blade 14 made of a plate-like member having a cutting edge 141 formed at its distal end, rockably mounted via a pivotal shaft 13 fixed in a direction perpendicular to the shaft 11 in the space 110 of the shaft 11 and protrusion parts 14a and 14b of the distal and rear ends respectively formed at the distal and rear ends in a radially inward direction.