PURPOSE: To realize a compact structure and allow the application thereof to an earth drill or the like by eliminating local load concentration via the sharing of resistance against a load from each direction with each section of the engaging plane of a holder and a bit.

CONSTITUTION: Recess 2 and 2, a division 3 and a widened and extended projection 5 are formed in a holder 1. Also, a drilling bit 6 is fitted with tail plates 7 and 7 so sized as to be housed in the recesses 2 and a groove 8 for the insertion of the division 3. Furthermore, such a shaft hole 10 as corresponding to the tail plates 7 and division 3 are provided and the holder 1 and the bit 6 are mated, engaged and joined to each other with a bolt 11 or the like. According to this construction, resistance against a force tending to rotate the bit 6 in a thicknesswise direction is shared by the engaged section of the projection 5 and the groove 8. As a result, no load acts on the tail plates 7 or the bolt 11, and a increase in the diameter of the plate 7 and the diameter of the bolt 11 is not required, thereby realizing a compact structure.