This study investigated the H-reflex and reciprocal Ia inhibition during fatigue in the human soleus muscle. Ten healthy subjects participated in this study, and performed intermittent isometric voluntary contraction of the ankle plantarflexion at 50% MVC as the fatiguing task. Reciprocal Ia inhibition was evaluated by the degree of H-reflex amplitude depression in the soleus muscle by the test stimulus following conditioning stimulus to the common peroneal nerve. The difference in H-reflex amplitude between before and after fatiguing task was also checked. There was no significant difference in the degree of H-reflex amplitude depression, although the H-reflex amplitude significantly decreased after the fatiguing task (p < 0.01). From the results of this study, it was considered that the decrease in H-reflex amplitude was caused by descending inhibitory input from the supraspinal to alpha-motoneuron, and the excitability of the Ia inhibitory interneuron was not involved. It was suggested that the function of reciprocal Ia inhibition was difficult to modulate during fatigue caused by isometric voluntary contraction in this study.