Plurality of users share a common key while permitting change of members sharing the common key and computational complexity required for key exchange is reduced. Ri and ci are computed based on a twisted pseudo-random function in a first key generation step. sid is generated based on a target-collision resistant hash function and (sid, Rα, Rβ) is transmitted to communication devices Ui in a session ID generation step. T1 and T′ are computed based on a pseudo-random function in a representative second key generation step. Tj is computed based on the pseudo-random function in a general second key generation step. k′ is computed based on the twisted pseudo-random function and T′j is computed with respect to each j in a third key generation step. K1l and k1 are computed in a first session key generation step. A common key K2 is generated based on the pseudo-random function in a second session key generation step.