Even when temperature sensors (12, 13) used in a device for warming a battery (1) being not in use by using a battery-driven heater (2) have failed, the battery (1) is prevented from freezing. Based on a battery temperature (Tb) and an outside air temperature (Ta), times (Δt1 to Δt8) during which Tb will decrease down to a warming start temperature (Tb_start) are each set to the next controller startup time (Δt). A controller (9) is started up every Δt, at t2, t3, and t4, and checks whether Tb