Natural human faces with abnormal visual features produce uncomfortable impressions, but artificial faces (e.g., robotic faces) do not necessarily do so. This is an example of the phenomenon called the uncanny valley. We hypothesized that this phenomenon indicates that natural and artificial faces are processed by different perceptual mechanisms, or they are processed differently by common mechanisms. We tested these hypotheses using a facial aftereffect where prolonged observation of adaptation faces with enlarged eyes induced a bias to underestimate the eye size of test faces. The results showed that adaptation to natural stimuli induced the aftereffect for both natural and artificial test stimuli. This suggests that the two types of faces engage common perceptual mechanisms. Adaptation to artificial stimuli also induced the aftereffect for natural test stimuli. However, artificial stimuli required a longer adaptation period (120 s) for the aftereffect to be induced compared to natural stimuli (60 s), suggesting that the processing of artificial faces by the human visual system may be inefficient. The uncanny valley may reflect that artificial faces are processed inefficiently by perceptual mechanisms that are common for processing natural and artificial faces.