Abstract: Validation of an input protocol is divided into two steps. In the first processing step, a state. transition graph of each process including only executable state transitions of each process is generated according to transition information which the protocol specification has, and at the same time, protocol errors related to behavior of the process is detected. In the second processing step, reachable system states among system states defined by combinations of individual process states and individual channel states are successively generated according to transition information which the state transition graph of each process has, and a system state of the generated reachable system states which cannot transit further is detected as a deadlock. In the protocol validation method, every system state can be erased from the memory when a deadlock detection for it is completed, and searching the generated system states is not necessary, and some additional techniques for more efficient validation can be applied.