A reinforcing bar binding machine is provided with: a main sleeve (11) having a tip end on which a hook (10) is pivotally mounted; a tip end shaft (12) fitted in an inside of the main sleeve (11); a spiral screw groove (14) formed on the tip end shaft (12); a fitting opening (13) that penetrates from an outside to the inside of the main sleeve (11); a key (15) fitted in the fitting opening (13) and brought in mash engagement with the screw groove (14); a short sleeve (16) provided on an outer periphery of the main sleeve (11) and covering the key (15); and an engaging means (33,34) formed on the short sleeve (16) and controlling a rotation of the main sleeve (11). Furthermore, a reinforcing bar binding machine is provided with a sleeve (11,16) having a tip end on which a hook (10) is pivotally mounted and a shaft (12) provided with a jutting part (27) on its base portion, sleeve (11,16) and shaft (12) being in engagement through a key (15) and a spiral screw groove (14) formed on the shaft, and a bumper (42).