This deformation-detecting sensor for a sealed secondary battery is used with a sealed secondary cell, battery module, or battery pack and comprises the following: a magnetic resin layer attached to the cell or battery module; and a magnetic sensor mounted to the inside or outside wall of an outer case so as to be able to detect changes in the magnetic field produced by the magnetic resin layer. The magnetic resin layer comprises a magnetic filler dispersed in a macromolecular matrix. A deformation-detecting sensor that can detect bulging of a sealed secondary battery with higher sensitivity and exhibits stable detection characteristics can thus be provided.