To improve safety by reducing risk of water immersion although an electric spare power-storage-unit connected with an electric battery in parallel is placed below a seat inside a cabin.

An onboard electrical spare power-storage-unit 2 is connected in parallel to an electric battery 1 for supplying electric power to a vehicle load 61 mounted on a vehicle. The electric spare power-storage-unit 2 includes plural power-storage devices 4 and a housing case 3 for housing the plural power-storage devices 4. The housing case 3 can be placed below a seat 13 in a cabin 12 of a vehicle. And a spacer 5 is provided to establish a spacing 16 between the lower surface of the housing case 3 and a floor surface 14 of the cabin 12.