English is a language with a parameter of strict verb-object adjacency. In this study research was made on the acquisition of the adjacency, targeting at the difference between Japanese learners of two different proficiency levels and native speakers with respect to three factors: grammatical compe-tence, linguistic knowledge, and linguistic performance. The results clearly show the difference be-tween Japanese learners and native speakers: regarding the reaction time for answering the grammaticality judgment task correctly, native speakers are not affected by the sentence kind, i.e. grammatical or ungrammatical, nor by the number of adjuncts inserted between verb and object. However, regarding the accuracy of answers, the differences between the Japanese and native speak-ers are not so big as those on the reaction time in both the sentence kind and the number of adjuncts. The results of this study suggest that research on the language acquisition should include not only linguistic knowledge but also linguistic performance in its models, and that it be necessary to design teaching and evaluating methods attaching great importance to the reaction time for processing and judging English sentences, which hints at what should be improved in the teaching of English in Japan.