PURPOSE: To carry out the balance keeping with high precision and reduce the working manhour by fitting and fixing a plate on which a weight for balance which corresponds to the imbalance quantity portion is added, into the groove on a rotary body so that the optimum position relation is obtained.

CONSTITUTION: When balance keeping for a main sheave 20 is performed, a balance keeping plate 25 is fitted in a circumferential groove 21 on the side surface of the sheave body 20, and the plate 25 is installed on the sheave body 20 by a stud 35, nut 38, etc. After, on a jig for detecting imbalance, the imbalance quantity and the imbalance position of the sheave body 20 are detected, the balance keeping plate 25 is demounted from the sheave body 20, and a balance weight 30 is installed on the balance keeping plate 25 by a bolt 31, nut 33, etc., at the 180 symmetrical position with respect to the imbalance quantity position. In the final stage, the balance keeping plate 25 on which the weight 30 is installed again is installed at the position where the imbalance of the sheave body 20 is detected, and the balance keeping work for the main sheave 20 is completed.