A lithium ion battery comprises a positive electrode terminal and a negative electrode terminal, a battery case, and an electrode body housed inside the battery case. The electrode body has a positive electrode current collector and a positive-electrode electrode plate and a negative electrode current collector and a negative-electrode electrode plate, and has a structure in which the positive-electrode electrode plate and the negative-electrode electrode plate are laminated via a separator. A carbon-based porous material in a battery case has a methane gas absorption capacity and preferably has a carbon dioxide gas absorption capacity and has a pore volume of 0.3 mL/g or less, for pores having a pore size of 4.5 Å or more. Such a gas absorber for a lithium ion battery generates no gas from reaction with an electrolytic solution of the lithium ion battery and can suitably absorb methane gas generated at the time of abnormality.