It has been shown that injection of a minute electrical noise to an afferent input enhances the steadiness of force output, while reducing the variability of motor unit discharge intervals. To elucidate the involvement of alpha motor neurons in this noise effect, the effect of electrical noise injection into the soleus muscle (SOL) on the excitability of the alpha motor neuron group was examined. Nine young human subjects lying supine on a bed received a minute, subthreshold electrical white-noise-like stimulation to the SOL. In each subject, a 20-min trial with noise (Noise) and another 20-min trial without noise as control (Con) were performed in random order. The H-waves and M-waves were successively elicited at 1 Hz during each trial. The results showed that the static properties (mean, standard deviation, and coefficient of variation) of H-wave and M-wave amplitude fluctuations were not different between Noise and Con conditions (p > 0.05). On the other hand, the scaling exponent α, a dynamic property of the fluctuation of H- and M-wave amplitude, decreased significantly under Noise condition in H-wave amplitude (p < 0.05), but was unchanged in M-wave amplitude (p > 0.05). These results indicate that application of electrical noise to the afferent input affects the dynamic properties of motor neuron excitability, thus supporting the notion that the motor neurons are involved in the effect of noise injection on the motor control system.