An optical network system including an OLT and ONUs is provided that can prevent the loss of a multicast signal. When receiving an encryption key generation request from the OLT, the ONU generates an encryption key, and transmits the generated encryption key to the OLT. When receiving a notice of timing from the OLT, the ONU updates the encryption key of a belonging group. When receiving a report message from a STB through the ONU, the OLT analyzes the report message, stores a group that the STB belongs to as well as the ONU in a second table, and transmits the encryption key generation request to the ONU. When receiving the encryption key from the ONU, the OLT further stores the encryption key in the second table, and transmits to the ONU a notice of the timing in which the encryption key is valid.