It is a relatively known fact that children acquiring Japanese as a first language (L1)tend to overuse "NO" as in "*ookii no kuruma" instead of the correct form, "ookii kuruma(=a big car)". This overgeneralization is also reported to have been observed in the utterances of adult learners of Japanese as a second language (L2).There are two different opinions about this phenomenon. First, this is one of the developmental errors so that learners make mistakes in spite of their native languages. Second, it could be the possibility of language transfer from the Chinese language. This paper discusses these two different opinions by conducting an investigation includ-ing 22 adult learners of Japanese learning in Japan. They were interviewed individually twice, at the beginning and at the end of the course. Each interview took about thirty minutes. These are the findings; (1) Some learners take "unit formation strategy", one of the language processing strate- gies, in the overuse "NO". (2) For Chinese learners, more errors are found by the end of the semester of the ad- vanced level compared to the beginning of the intermediate level. (3) Chinese learners overuse" NO" as the modifier for all I-adj., Na-adj. and verbs. The study concludes that (1)shows some of the errors could not be caused by language transfer and it supports the first hypothesis. On the other hand, (2)and(3)show the evidence that might be caused by transfer from their native language, Chinese, which supports the second hypothesis.