A vehicle master device includes a rewrite target specifying unit that is configured to identify a plurality of rewrite target ECUs, a rewrite completion determination unit that is configured to determine whether a program rewrite is completed for all the plurality of rewrite target ECUs specified by the rewrite target specifying unit, an activation executable determination unit that is configured to determine whether activation is executable when the rewrite completion determination unit determines that the program rewrite is completed for all the plurality of rewrite target ECUs, and an activation request instruction unit that is configured to instruct all the plurality of rewrite target ECUs at the same time to perform the activation when the activation executable determination unit determines that activation is executable.