An authentication method for link protection between an OLT and an ONU newly connected thereto in an EPON, which is implemented in a data link layer to which cryptography is applied. First, an authentication key is distributed to both the OLT and an ONU. The OLT (or ONU) generates first and second random values, generates an authentication request frame containing the random values, and transmits it to the ONU (or OLT). The ONU generates a first hash value according to a hash function using the random values contained in the request frame, and transmits an authentication response frame containing the first hash value to the OLT. The OLT compares the first hash value with a second hash value calculated by it according to the has function using the two random values and an authentication key distributed to it, and transmits an authentication result frame to the ONU.