A gateway ECU includes an update condition table that indicates a vehicle load state capable of updating an ECU program corresponding to each of several ECUs. The gateway ECU wirelessly communicates with an external center apparatus to receive an update file. The gateway ECU uses the update condition table to determine whether the current vehicle load state equals a lightly loaded state capable of updating an ECU program or a heavily loaded state incapable of updating an ECU program. If the current vehicle load state is determined to equal the lightly loaded state, the gateway ECU updates an ECU program using the update file received from the center apparatus. If the current vehicle load state is determined to equal the heavily loaded state, the gateway ECU performs environment improvement control based on the update condition table to change the current vehicle load state to the lightly loaded state.