Excavating tool

An excavating tool comprising:

• a casing pipe that forms a cylindrical shape about an axis line and in which a stepped portion whose inner diameter is decreased by one step is formed in an inner peripheral portion of a distal end;
• an inner bit which has a contact portion which can come into contact with the stepped portion on an outer periphery, and is inserted into the casing pipe from a rear end side in the direction of the axis line to protrude its distal end portion from a distal end of the casing pipe;
• an engagement convex portion that is disposed on the outer periphery of the distal end portion of the inner bit so as to be retractable;
• a ring bit that forms an annular shape and is arranged around the distal end portion of the inner bit protruding from the distal end of the casing pipe;
• and
• an engagement concave portion that is formed in an inner peripheral portion of the ring bit,
• wherein in such a manner that the engagement convex portion protrudes to an outer peripheral side and engages with the engagement concave portion, the ring bit is rotatable around the axis line integrally with the inner bit in a rotating direction during excavation, and the ring bit is locked so as not to be pulled out to the distal end side of the inner bit in the direction of the axis line,
• wherein the engagement convex portion is caused to retreat to an inner peripheral side, thereby the ring bit is capable of being pulled out to the distal end side of the inner bit, and
• wherein an outer diameter of the engagement convex portion is smaller than an inner diameter of the casing pipe.