Provided is a power storage device with superior safety that can detect swelling of the outer cover material at a low cost with a simple constitution. The power storage device (1) comprises a storage element (10), outer cover material (20) that accommodates the storage element (10), and a deformation sensor (30) disposed on an expansion surface (21) of the outer cover material (20). The deformation sensor (30) comprises a sensor film (32) which has a matrix formed from an elastomer or a resin and a conductive filler which is put into that matrix and which bends and deforms with expansion of the outer cover material (20) and at least one pair of electrodes (33a, 33b) connected to the sensor (32). A three-dimensional conductive path is formed in the sensor film (32) by the contact among the pieces of the conductive filler. Electrical resistance increases with the increase in the amount of deformation of the sensor film (32) from a natural state. The storage device (1) detects the swelling of the outer cover material (20) by changes in the electrical resistance that accompany bending and deformation of the sensor film (32).