We have discovered that electrochemically synthesized polypyrrole films undergo quick and intensive bending in ambient air without the use of heat, ions, or an electric field. The principle of motion was based on a reversible van der Waals adsorption of water or polar organic molecules onto the film, which distinguished it from other systems owing to electrochemical doping and undoping. The motion of film largely depended on the kind of adsorbate used : water vapor molecules caused rapid bending of the film to the opposite side. In contrast, when polar organic molecules were used as an adsorbate, the bending of film occurred to the same side, namely, the direction of bending was just opposite to that observed for water vapor. On the basis of this phenomenon, we have devised a novel actuator which moves by a spontaneous rotation. Further, an experimental "polypyrrole engine" has been made, which used a polypyrrole belt as working substance and adsorbate as fuel. When water and iodomethane were used as the adsorbates, the belt rotated at a speed of 22 cm min(-1).