The characteristic motion of a 1,10-phenanthroline grain on an FeSO4 aqueous phase was investigated as a simple example of an autonomous motor. The uniform motion of the grain changed to intermittent motion (periodic change between motion and rest) and its period and resting time increased depending on the concentration of FeSO4 (C-FeSO4). However, this intermittent motion reverted to uniform motion and the velocity increased with a further increase in C-FeSO4. The nature of this characteristic motion depending on C-FeSO4 is discussed in relation to the surface tension, which depends on surface concentration of surface active layer composed of 1,10-phenanthroline and a tris(1,10-phenanthroline) complex as the driving force.