A reaming section (13) is provided at the front end of a bit body (11) which is rotated about an axis (O), and the reaming section (13) has a greater diameter than the rear end of the bit body (11). Digging tips (15) are arranged at the front end of the reaming section (13). Earth discharge grooves (17) extending in the direction of the axis (O) are formed in the outer periphery of the reaming section (13). Connection grooves (18) connecting to the earth discharge grooves (17) are formed so as to extend from the outer periphery of the reaming section (13) to the rear end.
