Polypyrrole films containing tetrafluoroborate were electrochemically synthesized, and stress generated in the film under application of an electric voltage was measured in air under various strains. It was found that the film generated contractile stress in response-to the applied voltage that reached 6.1 MPa under 2 V, which was 4 orders of magnitude larger than its own weight. The mechanism was based on a dimensional change of the film that was expressed by two processes both caused by the Joule heating: one was the contraction due to the desorption of water vapor, and the other was the thermal expansion of polymer chains, which was different from electrochemical or chemical doping in an electrolyte solution or in a redox gas atmosphere. The generated stress under 2 V increased to 8.9 MPa upon stretching the film by 1%, which could be associated with the Young's modulus of the film rose due to the desorption of water vapor that plasticized polymer chains. The work capacity of the film increased as the applied voltage became higher and reached 48.2 kJ m(-3) at 3 V,: while the energy efficiency, defined as the ratio of work capacity to the electric energy, was on the order of 10(-3)%.