PURPOSE: To provide a slip quantity measuring device for a main cable by which the slip quantity of the main cable can be measured with a high precision.

CONSTITUTION: A slip quantity measuring device for a main cable is equipped with a position detecting means 10 which detects the initial position and the position after transfer of a sheave 1 when a main cable 2 transfers by a specified distance from the first standard position to the second standard position, memory means 11 for memorizing the detection value obtained by the position detecting means 10, and a slip quantity calculating means 12 which calculates the position change quantity for the sheave 1 by calculating the detection value memorized in the memory means 11 and calculates the slip quantity of the main cable 2 on the basis of the position change quantity. Accordingly, the slip trouble of the main cable can be surely prevented.