Transmission apparatus, transmission-reception apparatus, communication system, and communication method

In a vehicle-side transmitting apparatus 10 that applies a multi-link system employing a logical plurality of communication lines and includes a transmitting unit that transmits transmission data that should be transmitted to a receiving apparatus of a communication partner, the transmitting unit monitors changes in transmission rates of communication lines A to C, calculates, based on rated transmission rates, information concerning changes in the communication lines A to C, and a transmission time until data that should be transmitted to a ground-side receiving apparatus 20 is completed to be transmitted to the ground-side receiving apparatus 20, fluctuating transmission rates of the communication lines A to C , and determines, based on the calculated transmission rates, allocation amounts to the communication lines of data that should be transmitted this time.