Abstract: According to one embodiment, an electrical connection box includes: a base unit including a common circuit to be used in common to multiple vehicle models or grades; multiple kinds of option units respectively including an addition circuit selectively usable according to vehicle models or grades and, wherein: the common circuit of the base unit and the addition circuit of one of the option units cooperate together to form an electric circuit to be connected to a load provided on a vehicle body by combining the one of the option units with the base unit; and the base unit and the one of the option unit are mounted on a case of synthetic resin prepared for every vehicle model or grade and are then fixed to the vehicle body.