A user communication terminal (TEb14) updates encryption information when another user communication terminal (nTE113) leaves a group, by using terminal individual information of only communication terminals (nTE213, nTE313) remaining in the group (PNy). The communication terminal (TEb14) uses the updated encryption information to encrypt a shared key in PN (KPNy). The communication terminal (TEb14) transmits encrypted shared key information obtained by the encryption to a management device (PNSP11) to update encrypted shared key information stored in the management device (PNSP11).
