Relation of a close anatomical conjugation between the parietal association area and the premotor cortex underlies visuomotor integration. Numata et al confirmed activation of the anterior intraparietal sulcus (aIP) during action observation by a fMRI study, but they did not find a direct relation between aIP and the premotor cortex. In this study, the excitability of the left and right primary motor cortex (M1) (the first dorsal interosseous muscle controlling area) in a resting state during observation of adduction and abduction movements of a unilateral index finger displayed on a monitor were investigated in 11 healthy subjects using two transcranial magnetic stimulators (interstimulus interval, 10 ms). The result indicated an increased excitability of the contralateral M1 and an increased suppression of the ipsilateral M1 in the acion observation condition compared with the condition without action observation. The increased contralateral M1 excitability may be attributed to information transmitted from aIP via the premotor cortex, while the increased suppression to the ipsilateral M1 may be attributed to an increased interhemispheric inhibition (IHI) associated with the increased contralateral M1 excitability. Action observation is suggested to be a possible effective means to correct the imbalance of IHI occurring after brain injury and facilitate recovery from paralysis.