A battery monitoring apparatus monitors a state of a nickel hydrogen battery. The apparatus includes a board having a high voltage part having a voltage detection section detecting a voltage of the nickel hydrogen battery, and a low voltage part operating at voltage lower than voltage of the high voltage part, a housing that accommodates the board, and an attachment member that is attached to an opposed portion of the housing, which is opposed to a board surface of the board to attach the housing to a predetermined position. A passage is formed between the opposed portion and the attachment member. The passage extends from an opening part provided at a circumference part of the housing and along the opposed portion. The passage is located at a position that overlaps with the low voltage part, when viewed in a normal direction of the board surface.