The present disclosure aims to provide a communication system capable of achieving advanced security in a 5G communication system. The communication system according to the present disclosure includes: a communication terminal (10); an Access and Mobility Management (AMF) entity (20) configured to execute Mobility Management (MM) processing regarding the communication terminal (10); and a Session Management Function (SMF) entity (30) configured to execute Session Management (SM) processing regarding the communication terminal (10), in which the communication terminal (10) sends an MM message used in the MM processing, a first security key having been applied to the MM message, between the communication terminal and the AMF entity (20), and sends an SM message used in the SM processing, a second security key having been applied to the SM message, between the communication terminal and the SMF entity (30) via the AMF entity (20).