The aim of the current study was to clarify how corticospinal excitability (CSE) is modulated during the observation of the action of walking, combined with motor imagery (MI). We examined the difference in phase modulation of CSE with and without MI during the observation of the action of walking. We measured motor-evoked potentials in tibialis anterior and soleus muscles using transcranial magnetic stimulation under the following conditions: (1) baseline, (2) action observation (AO), and (3) AO and MI. Our results showed that, compared with AO alone, AO and MI of walking increased motor-evoked potentials. Our results suggest that CSE may be facilitated during AO and MI of walking, rather than during AO alone. Further, we postulated that AO and MI of walking may facilitate CSE in the tibialis anterior in a manner such as that evoked during actual walking.