ERP (Event Related Potential) measurement using the violation paradigm of Japanese verb conjugation was conducted in order to investigate the mental and neural mechanisms involved in the processing of different conjugation patterns. A LAN-like component followed by a P600 was elicited for the anomaly of using a tense-bearing form with the negative ending, while only P600 was observed for the anomaly of using an infinitive form in the same environment. The non-application of morpho-phonological changes of verb roots (“onbin”) yielded an N400 component and a P600. The P600 components observed in all types of errors reflect the cost of processing morphological and/or syntactic anomalies, while the difference in the negativities suggest that two different mechanisms of rule-based computation and lexical memory are involved in the processing of Japanese verb conjugation.