The author should regard modality as “possibility” in Japanese modality, based on the category setting that is further based on the so-called “-yes /- no standard” of SFL (System Functional Linguistics), and the uncertain area that exists between the affirmation and denial of “whether or not it is realized”. Therefore, the author regards “possibility” in Japanese modality as largely “ability possibility” and “situation possibility”, Additionally, “situation possibility” is further subdivided into “conditional situation possibility”, “atribute situation possibility”, and “spontaneous situation possibility”.The author believes that it is necessary to list four Japanese words (common language): “-rareru”, “-eru”, “-dekiru”, and “-surukotoga dekiru”.