10. A non-transitory computer readable storage medium having stored therein a document checking program, the document checking program causing a computer to execute a process comprising: extracting character strings each of which includes a predetermined word or a predetermined phrase from a target document and a confidential document that becomes a comparison target when determining similarity between documents; first determining, based on appearance positions of the character strings extracted, whether another character string is extracted within a predetermined range extending from a target character string toward either front or rear of the documents at the extracting; generating, when it is determined that the another character string is extracted within the predetermined range, a feature element matrix having a specific value in a matrix element at a row and a column corresponding to a pair of hash values of the target character string and the another character string within the predetermined range; comparing the feature element matrix of the target document and the feature element matrix of the confidential document; and second determining whether a number of common elements having the specific value commonly among the feature element matrices of the target document and the confidential document is not less than a first similarity determination number to determine similarity between the target document and the confidential document on the basis of the determination result.
10. A non-transitory computer readable storage medium having stored therein a document checking program, the document checking program causing a computer to execute a process comprising: extracting character strings each of which includes a predetermined word or a predetermined phrase from a target document and a confidential document that becomes a comparison target when determining similarity between documents; first determining, based on appearance positions of the character strings extracted, whether another character string is extracted within a predetermined range extending from a target character string toward either front or rear of the documents at the extracting; generating, when it is determined that the another character string is extracted within the predetermined range, a feature element matrix having a specific value in a matrix element at a row and a column corresponding to a pair of hash values of the target character string and the another character string within the predetermined range; comparing the feature element matrix of the target document and the feature element matrix of the confidential document; and second determining whether a number of common elements having the specific value commonly among the feature element matrices of the target document and the confidential document is not less than a first similarity determination number to determine similarity between the target document and the confidential document on the basis of the determination result. 12. The non-transitory computer readable storage medium according to claim 10 , wherein the second determining includes determining, when it is determined that the number of the common elements is not less than the first similarity determination number, whether an appearance density of the pair of the target character string and the another character string inside the target document is higher than a reference value, and specifying, when it is determined that the appearance density of the pair of the target character string and the another character string inside the target document is higher than the reference value, a part that has a high appearance density by the determination as a similar part similar to the confidential document inside the target document.