Provision of an extension-type battery pack which is improved in safety thereof against dropping and impact application. Although a battery pack is susceptible to damage caused by an impact since partial battery cells are located outside the outline of a notebook PC when it is mounted on the notebook PC, the battery pack is provided with a partition wall structure including an air space and a fireproof wall formed of a material which does not melt at a temperature of combustible gas discharged from a gas discharge valve of the respective battery cells. The material of the fireproof wall is preferably mica or the like. The partition wall structure can be provided between an inside battery compartment and an outside battery compartment of the battery pack or can be provided among other battery cells. This prevents fire spreading to other battery cells even if high-temperature combustible gas is discharged from a battery cell damaged by an impact or the like.