We have developed measuring system for arc duration time of commutation spark, which is fabricated to evaluate a commutation action of D-C machine quantitavely. The superior features of the developed system are as follows 1) It is enables measuring of the duration time of commutation spark all of the commutator segment. 2) Accuracy of the system is within 0.1 μs. with 0.5 ms. commutator period (maximum rotation number). 3) It makes possible the long time automatic measurement. We have measured the duration time of commutation arc to JEC-54 commutation spark number. Findings are: 1) The ratio of frequency distribution has two peaks to the duration time of commutation arc. 2) The maximum point of arc duration time in the ratio of frequency distribution increases according to the commutation spark number. 3) Arc duration time depends on brush materials and revoling speed with the same commutation spark number which is measured with eye. It suggests that evaluation of commutation spark with eye measurement is inaccurate. Hence, quantitave measurement is necessary.