Provided is an energy storage apparatus capable of appropriately controlling use of a silicon material in normal times and achieving long life, and a method of using the energy storage apparatus. One aspect of the present invention is an energy storage apparatus that includes an energy storage device and a measuring section for measuring an internal pressure change rate of the energy storage device, the energy storage device having a negative electrode that contains a carbon material and a silicon material. Another aspect of the present invention is a method of using the energy storage apparatus that includes performing discharge while the internal pressure change rate of the energy storage device is measured.