"The adverb 'yoku' has mainly three purposes of usage that are 'degree', 'frequency'', and 'evaluation', and the usage for 'degree' and 'frequency' are regarded as separate from the usage for 'evaluation'. These purposes of usage have been pointed out for their similarities with other adverbs, but on the other hand, it has been also pointed out that the difference in the usages were caused by the dilution of the original meaning and the difference in the degrees of the evaluation. In this article，we reviewed the 'evaluation' of the adverb 'yoku' that was common in various meanings. First，from the differences of the adverb 'yoku' from other adverbs，we extracted the common points throughout all the usages of 'yoku'. Then，by examining the characteristics of 'yoku's usage for 'degree evaluation'，we elucidated the structure of its 'degree evaluation'. Through these results，it was indicated that the basic meaning of adverb 'yoku' was the highness of the evaluation for the degree achievement attained against the set target. And it was elucidated that the one focuses on these standard is to be 'degree evaluation'，and the one focuses on the expectation upon speaking is to be an evaluation adverb'"