To extend a lifetime of a vehicular battery system by reducing temperature differences of respective battery cells through controlling of a temperature distribution of the whole battery cells in a simple structure.

The vehicular battery system has: a battery block 10 wherein a plurality of battery cells 1 are arranged in a laminated state; a cooling plate 7 arranged on respective battery cells 1 in a thermally-combined state; and a cooling mechanism 9 for forcibly cooling the cooling plate 7. The vehicular battery system has a first insulating layer 8 for limiting thermal conduction from the battery cell 1 to the cooling plate 7 between the battery cell and the cooling plate 7. An area of the first insulating layer 8 arranged between respective battery cells 1 and the cooling plate 7 is set up to be different for each battery cell 1 arranged in the laminating direction, and temperature differences of respective battery cells 1 are reduced by controlling thermal energy thermally conducted from the battery cell 1 to the cooling plate 7 on the basis of a difference of the area of the first insulating layer 8.

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