To provide an inter-LAN connection system that can effectively inform a communication company about a fault caused in a subscriber side in the case of connecting a user LAN to a broadband LAN of the communication company via an optical fiber transmission line.

When a subscriber side optical conversion repeater 40 detects link interruption between the user LAN 10 and the repeater 40, link interruption between the optical fiber transmission line 20 and the repeater 40 or power interruption, the repeater 40 generates an alarm packet shorter than a packet length of an Ethernet (R) frame respectively and transfers the alarm packet to an optical conversion bridge 50 of the communication enterprise. Upon the receipt of the respective alarm packets, the optical conversion bridge 50 outputs an alarm denoting a fault at the subscriber side by the respective packets externally. In this case, since the respective alarm packets are formed shorter than the usual frame and aborted by a MAC(Media Access Control) switch 5 after being supplied to a control section 512 of the optical conversion bridge 50, no alarm packet is delivered to the broadband LAN 30.