With development of internet and communication, electronic commerce is revitalized and the importance of digital document and digital signature are growing. Recently days, users hope to replace off line paper with electronic documents to efficient using these contracts, subscription documents and other documents should be stored during long-term which were created by electronic method. For this, we need to check stored documents whether it is the original copy or not through PKI system which proves the integrity of document check certificate validation and compare the value of singing with a genuine writing. There is a problem not to validate the document due to the period of certificate that can be used only 1 year. If the certificate were used for digital signature 3 years ago, we cannot validate certificate because it was already revoked. In order to solve this problem, we propose efficient long-term validation method of digital document even if certificate is expired.