A key management method, an encryption system, and a sharing digital signature system are provided, which are designed to hierarchically manage the degrees of contribution of members to a digital signature operation and the like so as to be suitably used for a group having a hierarchical structure. In an information communication system including a plurality of information processing apparatuses connected to each other through a communication line, a first member (11a) holding a secret key K and a plurality of second members (11b-11d) holding pieces of partial information K.sub.1i (i=1, 2, . . . ) generated by secret sharing of the secret key K are provided. The first member can use the secret key K as a key for the information communication system, but the second members can obtain a key for the information communication system only by collecting a predetermined number t.sub.1 or more of pieces of partial information K.sub.1i.