MONITORING SENSOR FOR SEALED SECONDARY BATTERY, SEALED SECONDARY BATTERY, AND MONITORING METHOD FOR SEALED SECONDARY BATTERY

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.