There is provided a turbine including a rotor wheel including a plurality of dovetail grooves and at least one insertion groove, the dovetail grooves and the insertion groove arranged in a circumferential direction of the rotor wheel; a plurality of first buckets, each of the first buckets including a vane, a platform provided at a first end portion of the vane, and a dovetail provided at the platform and having a shape corresponding to a corresponding dovetail groove; at least one second bucket including a vane, a platform provided at a first end portion of the vane, and at least one protrusion portion provided at the platform and configured to be inserted into a corresponding insertion groove of the insertion groove; and a fixing member configured to be inserted through the protrusion portion and the rotor wheel and configured to restrict radial movement of the second bucket.