A recent event-related potentials (ERP) study revealed that aspectual mismatch contexts evoked a significantly greater frontal negativity between 400-700ms. Another magnetoencephalography (MEG) study revealed that the aspectual mismatch is associated with increased activities of the anterior midline field (AMF), localized in ventromedial prefrontal cortex. In this study, we conducted an electrophysiological study of real-time computation for Japanese aspectual mismatch using ERP. We recorded ERP response to the sentences that included aspectual mismatch. The result indicated that the N400 component associated with semantic anomaly was not elicited in the aspectual mismatch condition, suggesting that semantic computation for an aspectual mismatch is different from that of a semantic mismatch between words and their discourse context. Additionally, a significant negativity was elicited in the 350-750ms time-window in the left anterior site. Considering the scalp distribution, this negativity for the coercion condition is likely to be a LAN effect, suggesting that aspectual coercion might enhance the cortical activation related to semantic ambiguity.