A battery heater device which heats a battery with a plurality of cells mounted on a vehicle comprises a first heater part which heats cells positioned at an outer side in the battery; a second heater part which heats cells positioned at an inner side in the battery; and a controller which controls power to be supplied to the first and second heater parts; wherein the controller reduces the power supplied to the second heater part at a predetermined timing after start of the power supply to the first and second heater parts, thereby improving quick-heating performance of the battery and heating the battery more uniformly with use of simple arrangement.