An in-vehicle communication system comprising: a gateway that is configured to relay communication between an external apparatus and an in-vehicle LAN; and a communication permission determination unit that is configured to inhibit communication between the external apparatus and the in-vehicle LAN through the gateway if it is determined that a vehicle having the in-vehicle LAN mounted therein is in a parking state based on acquired predetermined vehicle information, and to permit communication between the external apparatus and the in-vehicle LAN through the gateway if it is determined that the vehicle having the in-vehicle LAN mounted therein is not in a parking state based on acquired predetermined vehicle information, wherein: the communication permission determination unit is configured to determine whether an abnormality is detected in an acquisition source of the predetermined vehicle information; and if it is determined that an abnormality is detected in the acquisition source, the communication permission determination unit is further configured to permit communication between the external apparatus and the in-vehicle LAN through the gateway, characterized in that the communication permission determination unit is further configured to determine that the abnormality is detected in the acquisition source when a time during which the vehicle information is not received in a communication in which the gateway acquires the vehicle information exceeds a predetermined time. a gateway that is configured to relay communication between an external apparatus and an in-vehicle LAN; and a communication permission determination unit that is configured to inhibit communication between the external apparatus and the in-vehicle LAN through the gateway if it is determined that a vehicle having the in-vehicle LAN mounted therein is in a parking state based on acquired predetermined vehicle information, and to permit communication between the external apparatus and the in-vehicle LAN through the gateway if it is determined that the vehicle having the in-vehicle LAN mounted therein is not in a parking state based on acquired predetermined vehicle information, wherein: the communication permission determination unit is configured to determine whether an abnormality is detected in an acquisition source of the predetermined vehicle information; and if it is determined that an abnormality is detected in the acquisition source, the communication permission determination unit is further configured to permit communication between the external apparatus and the in-vehicle LAN through the gateway, characterized in that the communication permission determination unit is further configured to determine that the abnormality is detected in the acquisition source when a time during which the vehicle information is not received in a communication in which the gateway acquires the vehicle information exceeds a predetermined time. the communication permission determination unit is configured to determine whether an abnormality is detected in an acquisition source of the predetermined vehicle information; and if it is determined that an abnormality is detected in the acquisition source, the communication permission determination unit is further configured to permit communication between the external apparatus and the in-vehicle LAN through the gateway, characterized in that the communication permission determination unit is further configured to determine that the abnormality is detected in the acquisition source when a time during which the vehicle information is not received in a communication in which the gateway acquires the vehicle information exceeds a predetermined time.