A locking spacer, which is fitted in a dovetail slot provided on an outer circumferential surface of a disk put on a rotor shaft, includes: a pair of first blocks each provided with a dovetail joint, and configured to have a size occupying a portion of an internal space of the dovetail slot; a pair of second blocks having a size occupying a portion of the internal space of the dovetail slot, the portion not being occupied by the pair of first blocks, and each being provided with a locking groove; and a locking block having a size occupying a portion of the internal space of the dovetail slot, the portion not being occupied by the first and second blocks, and being provided with a rotating locking arm configured such that opposite end portions thereof are inserted into the locking grooves.