An auxiliary power storage device for an electric device installed in a vehicle, is connectable to a battery for the electric device in parallel to supply power to a vehicle load installed in the vehicle, and the auxiliary power storage device comprises a power storage portion and a housing case storing the power storage portion, and further the hosing case comprises a hole portion fitted to a terminal portion of the power storage portion, a mountain-shaped projecting portion integrally formed on a wall surface standing from a peripheral portion of the hole portion for preventing a liquid drop flowing down along the wall surface from intruding into the hole portion, and a projecting portion integrally formed along the peripheral portion of the hole portion for preventing a liquid drop from intruding into the hole portion from the peripheral portion of the hole portion.