We investigated the spatiotemporal patterns of event-related potentials to examine how semantic categories affect early stages of written word processing in the brain. We used a semantic priming paradigm in which the categories of prime and target words were the same (animate or inanimate). Event-related potentials were recorded while native Japanese participants made semantic-relatedness judgments of word pairs. Semantic category effects were observed approximately 150 ms after the target item was presented. Inanimate words elicited greater positive deflection than animate words, but no significant difference was observed for prime item processing. Source modeling estimated that the observed effects were induced by neural mechanisms occurring in the anterior temporal region, the posterior portion of the inferior temporal region, and the inferior parietal regions in both the hemispheres. Our results suggest that semantic category effects in the early stages of word processing are evoked by multiple cortical regions related to top-down processing of the preceding semantic information.