A battery 11 including plural unit cells 13 stacked on top of each other in the up-and-down direction is accommodated in a case 7 to constitute each battery module 1. The plural battery modules 1 are stacked on top of each other in the up-and-down direction to constitute a battery module stack 31. The case 7 includes a lower case 3 and an upper case 5. A sidewall portion 3b of the lower case 3 is situated outside of a sidewall portion 5b of the upper case 5, and an upper end 3b1 of the sidewall portion 3b is situated above a lower end 5b1 of the sidewall portion 5b. Between the sidewall portion 3b of the lower case 3 and the sidewall portion 5b of the upper case 5, a communicating portion 15 to discharge high-temperature gas generated within the case 7 to the outside is formed.