Realized is a vehicular battery monitoring device or a vehicular battery monitoring system for which the number of wires can be reduced, and that can transmit information using a characteristic transmission path. A battery monitoring device includes: a detection unit configured to detect at least one of a voltage at a predetermined position of a battery or a temperature of the battery; and a wireless communication unit configured to transmit, using a wireless communication method, detection information that indicates at least one of the voltage and the temperature of the battery based on a detection result of the detection unit, at least to another battery monitoring device provided in a vehicle.