In this paper, we propose a multi-party (group) key exchange protocol based on CSIDH (Commutative Supersingular Isogeny Diffie–Hellman), which is a post-quantum Diffie-Hellman type key exchange protocol from a commutative group action. The proposed group key exchange protocol called G-CSIDH uses the same size prime modulus p as that in CSIDH for the same security level, and the security of G-CSIDH is reduced to the security of CSIDH. In addition, we propose the trusted protocol of generating public parameters of supersingular isogeny cryptosystems by using the proposed G-CSIDH. Trust in the setup based on G-CSIDH is reduced to the security of G-CSIDH, and then that of CSIDH. The trusted protocol can be applied to any supersingular isogeny cryptosystem, which uses a supersingular elliptic curve as a public parameter.