A state-transition system includes a plurality of states and a first transition representing a state-change based on an external event occurred into a system to be checked. A plurality of requirement items includes a precondition and a postcondition in correspondence with the external event. By partially selecting the precondition and the postcondition from the plurality of requirement items, a state map to map a state onto truth values of the precondition and the postcondition is generated. By contracting the state-transition system with the state map, a quotient state-transition system is generated. The quotient state-transition system includes a plurality of second transitions among a plurality of groups each having states. By tracing each second transition in the quotient state-transition system, a representative transition path is generated. The representative transition path is regenerated as a transition path in the state-transition system. The transition path is output as a test case.