The self-motion of a solid scraping at an air/water interface was investigated under isothermal conditions. The self-motion of a camphoric acid scraping at an air/water interface changed characteristically depending on the pH of the aqueous phase. With an acidic aqueous phase, the scraping maintained a fairly constant velocity. At neutral pH, intermittent motion, the period of which depended on the pH, was observed. These characteristic motions are discussed in relation to the surface free energy which results when camphoric acid layer diffuses from the scraping. The essential features of the motion of a camphor scraping were reproduced by a computer simulation. These results imply chemo-mechanical energy transduction under isothermal conditions.