A blade lock structure for blading an axial flow turbine having a rotor and blades. The blades are inserted into an undercut blade groove of the rotor providing a positive lock between the blades and the rotor. The lock structure includes an entry slot located in the blade groove for receiving the blades for insertion to the undercut blade groove, and an insert space axially extending from the entry slot. The insert space defines a radially extending longitudinal axis. An insert piece is located in positive-locking relationship with the insert space, and a lock screw threadably engages between the insert piece and a closing blade inserted to the entry slot.