This paper proposes a Kripke frame that represents logical and empirical requirements with which the validity of planning action is verified. The frame is composed of a consistent set of states in planning and an equivalent relation among the states. It is shown that the frame is equal to S5 frame in modal logic. Therefore, it is possible to apply S5 axiom system to the proof of the validity of planning action. This paper also shows that the axioms and some theorems derived by the axiom system can be associated with the empirical understandings concerning the validity of planning actions.
