This paper presents a method to control the sway of a rotary crane by using a straight transfer transformation (STT) model. The STT model is built and its parameters are geometrically derived. Considering the variation of the rope length, optimal control of a rotary crane is solved using an optimization method of Davidon-Fletcher-Powell (DFP) so that the load has no sway at the end point of the transfer. The proposed control method using the STT model is demonstrated to be effective to reduce the influence of centrifugal force through simulation and experiments.