1. An electronic signature method executed by a computer, the computer executing the steps of: dividing a target electronic document into a plurality of document segments; generating a signature (s, t) that includes a set of two values having a signature value s forming a signature on the electronic document and a deletion signature value t used for deletion, the signature value s, which serves as a body of the signature, being formed by a superposition of signature information on the individual document segments; and superimposing, in a case where one of the plurality of document segments obtained by the division is to be extracted, deletion information of a document segment to be deleted on the deletion signature value t to generate a new signature value t′, and producing an updated signature (s, t′).