A deformation detection method for a sealed secondary battery, in which a polymer matrix layer is mounted in a compressed state in a form of being sandwiched in a gap, and the polymer matrix layer contains a filler that is dispersed therein and that changes an external field in accordance with deformation of the polymer matrix layer, the method comprising:
- detecting change in the external field accompanying the deformation of the polymer matrix layer, and detecting deformation of the sealed secondary battery on a basis of the detected change in the external field;
- and
- wherein
- the polymer matrix layer contains a magnetic filler as the filler, and
- the detection unit detects change in a magnetic field as the external field.