A vehicle master device is configured to acquire update data from outside and distribute the acquired update data to a rewrite target electronic control unit (ECU). The vehicle master device includes a distribution frequency specifying unit that is configured to specify a distribution frequency of the update data to the rewrite target ECU based on a correspondence relationship between a predetermined vehicle power supply state and transmission restriction information related to distribution of the update data and a vehicle power supply state during updating, and a distribution control unit that is configured to control distribution of the update data according to the distribution frequency specified by the distribution frequency specifying unit.