1. A method of detecting an original document from a plurality of web documents using a processor, the method comprising: receiving notification upon generation or modification of the web documents from a web server, the generation or modification of the web documents being detected by the web server when the web documents are cited as links in at least one web document in the web server; generating history information for a uniform resource locator (URL) of each of the web documents that are generated or modified, at a time the notification is received; crawling the web documents corresponding to the generated history information to determine whether the crawled web documents have been further modified since the time of receipt of the notification of generation or modification of the web documents; filtering, by the processor, the crawled web documents by excluding each of the crawled web documents that are determined to be further modified since the time of receipt of the notification; and detecting an original document of the crawled web documents that are filtered based on the history information; wherein the time of receipt of the notification also indicates a time the web documents are generated or modified; and a web document from the filtered web documents having an earliest generated or modified time is determined to be the original document.