Unmanned exploration by robots for volcanic environnements has been required. A tracked vehicle is one good candidate. When a tracked vehicle traverses volcanic environment, it faces tip-over because the ground in a volcanic environment is tilted and frequently covered with large rocks and a tracked vehicle climb over such obstacles on a slope. On the other hand, to increase the traversability, various multiple degrees of freedom tracked vehicles equipped with sub-tracks have been proposed. However, the relationship between tip-over condition on a slope and the sub-track's motion has not been sufficiently researched. The primary purposes of this study are to understand the tip-over phenomena for a tracked vehicle with sub-tracks climbing an obstacle on a slope, and to provide the optimal motion strategy. In this paper, the geometric tip-over condition for each sub-track angle and the motion strategy based on the posture at the moment of climbing over were considered. Moreover, experiments were conducted to validate the usefulness of the proposed method. From the results, the derived condition is valid if the robot does not slide down.