In this paper, I focus on the fact that Korean-speaking Japanese learners tend to use ‘niau’ when ‘au’ should be used. And the meaning of each word and whether or not they can be replaced were examined in order to help guide the proper use of ‘au’ and ‘niau’. As a result, it was found that ‘au’ begins with the meaning of “multiple things approach and gather”, and overall tends to express highly objective events. On the other hand, ‘niau’ means “feeling harmony or balance between superficial or external features” or “disagreement between the speaker’s preconceived notions about people or animals with certain features and their actual behavior”(used in the negative forms). Thus, it was found to represent highly subjective events. And it was revealed that ‘au’ and ‘niau’ are interchangeable, when they mean “feeling harmony or balance between superficial or external features”: the semantic usages that is the most subjective in ‘au’ and relatively low subjectivity in ‘niau’. In addition, it was revealed too that in the cases of the more objective semantic usages of ‘au’, and in the cases of the relatively highly subjective semantic usages of ‘niau’, they cannot be replaced, or acceptability of replacement is low.