A battery cooling system includes a depression-shaped battery accommodating section formed at a bottom surface of a floor panel of a vehicle and dented to side of a passenger compartment so as to accommodate therein a battery. An introduction device is disposed in front of the battery in a vehicle traveling direction, to introduce travel air flow into the battery accommodating section. Here, the battery is accommodated within the battery accommodating section in a condition where a bottom surface of the battery is located at a level above or at a substantially same level as a level of a bottom surface of the vehicle which bottom surface is around the accommodating section. Additionally, the battery includes a plurality of thin cells which are electrically connected with each other, and disposed in a posture that a major surface of each thin cell extends substantially parallelly with the vehicle traveling direction.