A battery pack comprising: battery modules, each of the battery modules including cylindrical batteries; thermal diffusing plates holding the batteries, wherein each of the thermal diffusing plates is a metallic plate provided with a number of through-holes into which the cylindrical batteries are inserted and the cylindrical batteries are held by the through-holes; and a first chamber into which cooling air to cool each of the cylindrical batteries can be introduced; characterized by comprising: a second chamber provided in outer spaces of the battery modules, at least part of a wall of the second chamber being configured by the respective thermal diffusing plates of adjacent battery modules; and a heater (35; 37) disposed in the second chamber, the heater (35; 37) facing the respective thermal diffusing plates of the adjacent battery modules across a predetermined distance, wherein the second chamber includes an air space, and the air space is disposed between the heater and the respective thermal diffusing plates of the adjacent battery modules and wherein the battery pack further comprises a flow passage, wherein the flow passage is configured to introduce the cooling air to the adjacent battery modules, the flow passage and the second chamber are disposed between the adjacent battery modules. battery modules, each of the battery modules including cylindrical batteries; thermal diffusing plates holding the batteries, wherein each of the thermal diffusing plates is a metallic plate provided with a number of through-holes into which the cylindrical batteries are inserted and the cylindrical batteries are held by the through-holes; and a first chamber into which cooling air to cool each of the cylindrical batteries can be introduced; characterized by comprising: a second chamber provided in outer spaces of the battery modules, at least part of a wall of the second chamber being configured by the respective thermal diffusing plates of adjacent battery modules; and a heater (35; 37) disposed in the second chamber, the heater (35; 37) facing the respective thermal diffusing plates of the adjacent battery modules across a predetermined distance, wherein the second chamber includes an air space, and the air space is disposed between the heater and the respective thermal diffusing plates of the adjacent battery modules and wherein the battery pack further comprises a flow passage, wherein the flow passage is configured to introduce the cooling air to the adjacent battery modules, the flow passage and the second chamber are disposed between the adjacent battery modules. a second chamber provided in outer spaces of the battery modules, at least part of a wall of the second chamber being configured by the respective thermal diffusing plates of adjacent battery modules; and a heater (35; 37) disposed in the second chamber, the heater (35; 37) facing the respective thermal diffusing plates of the adjacent battery modules across a predetermined distance, wherein the second chamber includes an air space, and the air space is disposed between the heater and the respective thermal diffusing plates of the adjacent battery modules and wherein the battery pack further comprises a flow passage, wherein the flow passage is configured to introduce the cooling air to the adjacent battery modules, the flow passage and the second chamber are disposed between the adjacent battery modules.