1. An authenticity assurance system that assures authenticity of an electronic document, wherein: the authenticity assurance system comprises: a management apparatus; a redaction-target document structure processing apparatus; a redactable signature method information processing apparatus; a redactable signature processing apparatus; and a signature file output apparatus; the management apparatus comprises a unit for inputting data to and outputting data from the redaction-target document structure processing apparatus, the redactable signature method information processing apparatus, the redactable signature processing apparatus and the signature file output apparatus; the redaction-target document structure processing apparatus comprises: a unit for inputting the electronic document from the management apparatus; a unit for converting the electronic document into a data string of at least one piece of data according to given area partitioning information; a unit for generating at least one area ID for identifying respectively the at least one piece of data constituting the data string; and a unit for outputting the converted data string and the at least one generated area ID to the management apparatus; the redactable signature method information processing apparatus comprises: a unit for inputting the converted data string and the at least one generated area ID from the management apparatus; a unit for generating a random number for each piece of data of the converted data string; a unit for calculating a hash value for data that are obtained by combining the converted data string and the generated random numbers; a unit for outputting the hash values; a unit for generating redactable signature method information that includes the at least one area ID and the random numbers or data capable of calculating the random numbers; and a unit for outputting the redactable signature method information to the management apparatus; the redactable signature processing apparatus comprises: a private key; a unit for inputting the hash value from the management apparatus; a unit for calculating a signature value from the hash value and the private key; and a unit for outputting the signature value to the management apparatus; and the signature file output apparatus comprises: a unit for inputting the area partitioning information, the redactable signature method information and the signature value, from the management apparatus; a unit for converting the area partitioning information, the redactable signature method information and the signature value into a signature file conforming to a given signature output format; and a unit for outputting the signature file to the management apparatus.