Provided is a turbine including a rotor wheel including a plurality of dovetail grooves and a insertion groove, the insertion groove including a first catching groove; a plurality of first buckets, each of the plurality of 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 the dovetail groove; a second bucket including a vane, a platform provided at a first end portion of the vane, and a protrusion portion and configured to be inserted into the insertion groove, the protrusion portion including a second catching groove; and a fixing member configured to be inserted into each of the first and second catching grooves, the fixing member configured to restrict radial movement of the second bucket.