6. The document checking apparatus according to claim 5 , wherein the second determining includes treating a predetermined number of consecutive character strings included in the target document as a determination part and determines whether a number of pairs of the target character string and the another character string inside the determination part is not less than the reference value, and the specifying includes specifying, when it is determined that the number of the pairs of the target character string and the another character string is not less than the reference value, the determination part as a similar part similar to the confidential document inside the target document.
6. The document checking apparatus according to claim 5 , wherein the second determining includes treating a predetermined number of consecutive character strings included in the target document as a determination part and determines whether a number of pairs of the target character string and the another character string inside the determination part is not less than the reference value, and the specifying includes specifying, when it is determined that the number of the pairs of the target character string and the another character string is not less than the reference value, the determination part as a similar part similar to the confidential document inside the target document. 7. The document checking apparatus according to claim 6 , wherein the second determining includes treating, when the similar part is not specified, a predetermined number of consecutive character strings starting from a character string next to a forehand character string of the determination part as a next determination part, treating, when the similar part is specified, a predetermined number of consecutive character strings ending at a character string corresponding to the common elements and located more backward than the specified similar part as a next determination part, and determining whether the number of pairs of the target character string and the another character string inside the next determination part is not less than the reference value.