The Japanese 'adverbial particle' wa is usually argued to have topic and contrastive functions. The previous analyses on contrastive function of wa have two problems. First, they fail to account for 'association with contrast' in complex NP. Second, the notion of contrast has not been made explicit enough to account for an asymmetry between contrasts involving and not involving wa. This paper assumes that the basic contrastive meaning is carried by phonological prominence, and argues that wa interacts with such a contrast resulting in a distinct presupposition, following 'alternative semantics' of Rooth.