I investigated problems in acquisition of case markers O and NI for advanced learners of JSL. I take the position that the case particle is the form of case indicating the relation to the verb, so, is governed by the verb. I picked out the "Mental Verbs" as the object of my study. The noun co-occurring to the mental verb takes O or NI, and at the same time, two of them belong to the objective in a broad sense. I predicted that the confusion caused by this fact should make it difficult for learners to acquire the use of case markers. Through the quiz, I obtained the following results: 1)Comparing O and NI co-occurring to the mental verbs with those to the physical verbs or the social verbs, former is lower percentage of right answers. 2)Fewer subjects chose the right answers to NI than those to O. 3)NI is frequently mixed up with O, and O with GA.
