A method for searching transition routes from an initial state of a plurality of stackable items to a target state by a computer at predetermined conditions includes connecting between transitionable states at an edge by a single handling operation transitioning from the initial state to the target state, to generate a state space graph; evaluating a lower bound of handling operation cost from a certain state to the target state, based on both the stacking sequence of the plurality of items in the certain state and the stacking sequence of the plurality of items in the target state; and searching a route from the initial state to the target state where the handling operation cost is minimal on the state space graph using a heuristic search algorithm based on the lower bound.