The key point of cryptography is cryptographic algorithms and keys. The random number generator is used to generate seeds and keys randomly in many cryptographic systems. For this reason, it is essential to use keys to encrypt and decrypt the transferring information, and the security of these keys is closely related to the security of 5G network. We propose a true random number generator (TRNG) method in this paper, and are using visible spectrum for noise source. We consider that if the cryptography system utilizes data of visible spectrum using the proposed TRNG, and the TRNG generates random numbers with high entropy.