Abstract: A plagiarism document detecting system and an automatic reference citation mark attaching system are provided which are capable of providing an on-line service and are based on a synonym dictionary. The automatic reference citation mark attaching system includes a memory on which at least one program is loaded and at least one processor. Based on a control of the program, the at least one processor performs operations of checking similarities between original sentences, included in an original document, and test sentences generated by dividing a to-be-inspected target document by the sentence; and providing bibliographic information of the original document as reference information on the test sentences when the similarities between the test sentences and the original sentences exceed a predetermined criteria.