A battery pack 1 of the present invention includes: a plurality of lithium ion secondary batteries 10 electrically connected together in series, in parallel, or in series-parallel combination; a tray 11; and a housing 12. The lithium ion secondary batteries 10 each include an electrode assembly containing an alloy-based negative electrode active material serving as a negative electrode active material, and each have a sealed surface 10a. The lithium ion secondary battery 10 is mounted on the tray. The housing 12 accommodates the tray 11 with the lithium ion secondary battery 10 mounted thereon. Even if high-temperature contents are produced in the interior of the lithium ion secondary battery 10 in the event of internal short circuiting or overcharging, this configuration prevents the high-temperature contents from leaking out of the batter pack 1, and thus improves the safety of the battery pack 1.