PURPOSE: To measure backlash of a speed reducer without stopping an elevator for long periods, as well as to shorten the time and to decrease labor required for the preparation and restoring work.

CONSTITUTION: A rotary encoder 6a capable of being brought in contact with the outer periphery of a sheave 1 is provided, and the contact timing of this rotary encoder 6a is limited to the time when an elevator is stopped, and the rotational displacement of the sheave 1 to be reversed by unbalance of a car and a balance weight, to be generated in getting on/off of a passenger is measured.