A Gigabit Ethernet passive optical network (GE-PON) for securely transferring data through exchange of an encryption key comprises an optical line terminal (OLT) for encrypting a secret key using a public key received through a transmission medium, transmitting the encrypted secret key, encrypting data using the encrypted secret key, and transmitting the encrypted data, and at least one optical network terminal (ONT) for transmitting the public key to the OLT, decrypting the encrypted secret key transmitted from the OLT using a private key, and decrypting the data encrypted with the encrypted secret key, transmitted from the OLT, using the decrypted secret key.