A Study on the Effect of Metaphor in Public Service Advertising. The study carried out a positivistic research on university students with a view to contributing to the building of a metaphoric ad strategy by analyzing the effect of metaphoric expressions in public service advertising, which showed the following: First, the metaphoric ads using positive approach and indirect copy were effective for heightened emotional attitude of viewers. Second, the metaphoric ads using positive approach and indirect copy were effective for heightened source credibility, which was further heightened when the two were simultaneously used. Third, the metaphoric ads using positive approach and indirect copy were effective for heightened design preference, and the design preference became even preferable when the two were simultaneously used. Fourth, the metaphoric ads using indirect copy were effective for behavioral intention. The study suggested that the strategy of adopting metaphoric ads with indirect copy of positive approach was quite useful in strengthening the effect of public service advertising.