To eliminate the need for exchange of even a bit holder and a cutter drum for exchanging a flat bit and a round bit, to improve workability and excavation efficiency at an excavation site and also to reduce the cost of the whole cutter drum device.

A rotation preventive surface 5 and a bit-shaft inserting hole 6 are formed to the base body 3 of a bit holder 2 fixed onto a cutter drum body 1. In the flat bit 11 inserted to the bit holder 3 in an exchangeable manner with the round bit, a bit shaft 16 inserted into the bit-shaft inserting hole 6 in a manner that the bit shaft 16 can be inserted and detached is projected from the rear surface of a bit parent material 12 with a cutting tip material 15, and an engaging projecting section 18 engaged with the rotation preventive surface 5 is formed. The round bit is composed of the bit parent material 12 having the cutting tip material 15 on the front end side and the bit shaft 16.