In this paper, an auto-positioning algorithm for sliding planes is newly proposed in two different forms: the General Virtual Force Algorithm (GVFA) and the Applied Virtual Force Algorithm (AVFA). The proposed algorithm is then applied to an auto-positioning spreader which can slide on the top surface of a container with 3 degrees of freedom (DOF). This enables the spreader to handle containers even on a wavy open sea, where the inevitable swinging motion of a spreader leads to significant misalignment from the container during landing. With numerical simulation and experiments using a 1/20 scale model, it is verified that the proposed algorithms provide a robust and reliable solution for in-plane path-finding. Considering the limited space and cost for sensor equipment, however, using AVFA with 8 sensors can be a better solution for an actual application regardless of the slight sacrifice in performance in terms of operation time and energy consumed.