Dodis et al proposed a key-insulated signature scheme in 2003. The scheme can minimize the damage caused by the secret key’s exposure but can not protect the user from the secret key’s exposure perfectly. We propose a PKI system which can detect immediately even a single illegitimate signature due to the exposure of a user’s secret key. The system uses the one-time hash chain based on NOVOMODO and can prevent the users from compromising the secret key more perfectly and effectively than the key-insulated signature scheme.