An event-related potential experiment was conducted in order to investigate readers' response to violations in the hierarchical structure of functional categories in Japanese, an agglutinative language where functional heads like Negation (Neg) as well as Tense (Tns) are realized as suffixes. A left-lateralized negativity followed by a P600 was elicited for the anomaly of attaching a Neg morpheme outside a Tns-marking suffix (i.e., syntactic violation of the form *[[V - Tns] - Neg]), while only P600 was observed for the anomalous form with a purely morphological/morpho-phonological violation, i.e., a Neg morpheme attached to "ren'yo" form instead of Neg-selecting form. The findings suggest that the syntactic structure involving Tns and Neg in Japanese, realized within a word as a sequence of suffixes, is processed in a similar manner to the syntactic structures that are phrasally realized in well-studied European languages like English.