For an experimental DC motor equipment immersed in ethanol and gasoline, commutator currents were measured at various rotational speeds to find out an arc duration time during commutation process and a residual current. The result revealed that the arc duration time for the ethanol was 3-6 µsec and this duration time was longer than that for the gasoline. The residual current in the ethanol also proved to be 2-3A and to be about twice higher than that in the gasoline. Subsequently these results were discussed on the basis of contact resistance just before brush-commutator separation. As a result, the contact resistance was found to be 0.6 times lower in the ethanol than in the gasoline. A novel approach was lastly made to the contact state of the brush on the commutator in view of mechanical properties: coefficient of friction, lubrication, contact load. This approach achieved successful explanation of the phenomena above.