Provided is a technique capable of detecting deformation based on an abnormality such as heat generation in an article by a simpler, smaller, or lower-cost configuration, to suppress occurrence of an accident due to the abnormality of the article. An abnormality detector for measuring deformation of a battery B, such as a lithium-ion battery, to detect an abnormality of the battery, the abnormality detector includes: a sealed container disposed in close contact with at least a part of an outer surface of the battery, while containing and sealing a fluid such as water, the sealed container having flexibility; and a pressure sensor configured to measure a change inside the sealed container, accompanying deformation of the battery.