This paper experimentally examines the effects of the case-markings and the constraint on the assignments and the receptions of thematic roles in Japanese sentence processing. A self-paced reading experiment was carried out with syntactically well-controlled Japanese sentences including homonyms locally ambiguous between nouns and verbs. The results showed that the homonyms were preferably disambiguated as verbs. We interpret this disambiguation as the result of the application of the thematic constraint to the input items on the basis of the correspondence between the case-markings and the grammatical functions in Japanese. We further examined the effect of pragmatic plausibility on the interpretation of the homonyms by questionnaire, and claim that the thematic constraint is still the chief determinant for their disambiguation even with the possible plausibility effects. We also examined the effect of the verbal working memory capacity estimated by the Japanese Reading Span Test, and we demonstrate that a reader with a high score in the test comprehends sentences more accurately but spends relatively longer time for reanalyses than a reader with a low score. We discuss the relevances of our results to the parsing models for real-time Japanese sentence processing and to the studies of verbal working memory in English. (Contains 1 table, 3 figures and 7 footnotes.)