An opening for a replacement bit can be closed without a large drive force, the replacement bit can be replaced without being moved, and the workability of replacement is not impaired. A rear surface frame (29) forms a workspace (28) on the rear surface side of a spoke front surface plate (21), and openings (33) are formed on the spoke front surface plate (21) such that a replacement bit (31) can be inserted into and removed from the opening (33). A bit guide body (37) having a bit guide hole (38) is provided on the rear surface side of the opening (33). A gate plate (34) is provided which has valve ports (35) capable of communicating and closing the openings (33) and the bit guide holes (38). A gate opening/closing device (36) is provided which can turn the gate plate (34) to open or close the openings (33).