A monitoring sensor for a sealed secondary battery 1 including a sealed outer casing 21 and an electrode group 22 accommodated in the inside of the sealed outer casing 21, including a polymer matrix layer 3 that is disposed in the inside of the outer casing 21 and a detection unit 4 that is disposed on the outside of the outer casing 21, wherein the polymer matrix layer 3 contains a filler that is dispersed therein and that changes an external field in accordance with deformation of the polymer matrix layer 3, and the detection unit 4 detects change in the external field.