The thermal runaway of a lithium ion battery (LIB) during a nail-penetration test was investigated using an LIB internal short circuit observation system equipped with an X-ray scanner (LiSC scanner). Using high-speed moving images and high-precision voltage measurements, the layer-by-layer internal short circuit caused by the nail was clearly observed during nail motion. Following this motion, gas generation outside the cell, which is well-known in thermal runaway, was observed. The main causes of smoke are speculated to be the boiling of the electrolyte and/or decomposition of the active materials owing to heating by the short circuit current. The initial behavior of the short circuit before gas generation was clearly observed. Therefore, gas generation, which is well-known to indicate an internal short circuit of the cell, and the electrical behavior of the short circuit during the nail-penetration test were observed separately. This LiSC scanner allowed us to analyze the details of the internal short circuit of the cell, and it is expected to lead to significant advancements in the safety of LIBs. (c) The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.