An acid liquid leakage sensor quickly detecting liquid leakage from a device, such as a battery or the like, containing an acid liquid includes a first conductive member, a second conductive member, and an electrically insulating material which establishes an electrically insulating state between these first and second conductive members. The electrically insulating material includes a macromolecular compound having a basic functional group, and an electrical insulation characteristic or high resistance characteristic that decreases upon reaction with an acid liquid. It is possible to detect leakage of acid liquid, such as battery fluid, by detecting the change of conductive state between the first conductive member and the second conductive member which accompanies a decrease of the electrical insulation characteristic or high resistance characteristic of the electrically insulating material, when acid liquid leaks out and drips down the electrically insulating material.