An in-vehicle communication system is disclosed. In the in-vehicle communication system, when a portable device has received a data request specifying a specific data from an in-vehicle apparatus, the portable device determines whether the portable device can receive the specific data from a center apparatus. When the portable device determines that the portable device cannot receive the specific data from the center apparatus and that the specific data is stored in a data storage area of the portable device, the portable device transmits the specific data stored in the data storage area to the in-vehicle apparatus.