PURPOSE: To provide a guide sheave mounting device which is designed to simplify the change of the height of the guide sheave of an upper boom without needing an auxiliary crane and shift a transport posture and a work posture.

CONSTITUTION: A support shaft 14 to support a guide sheave 7 is attached to a support bracket 16, and a support pin 17 the center of which is vertically displaced from the center of the support shaft 14 is fixed to each of the right and left outside parts of the support bracket 16. The support pin 17 is inserted in the hole of a fixed frame body 18 arranged to an upper boom 4 and a fixed frame body 18. The fixed pin 19 is pulled out and by rotating the support bracket 16 in a 180° arc, the height of the guide sheave 7 is changed.