To obtain the fundamental data at a construction of Man-Machine system, we examined the reaction time (RT) of finger to the visual stimulation. The light of LED was provided to the subject at the period of 3 second. The statistical parameters (average, standard deviation, histgram, auto-correlation and its spectrum) were computed from the observed RT. The repetition of the experiment did not change these parameters, The RT histgram was classified into three types for examined subjects so far. The auto-correlation and its spectrum showed the characteristics of white noise.