Robot-assisted therapy has gained attention as one of mental health measures. In this paper, we investigated the influences of soft tactility on psychological and physiological stress relief effects to verify whether soft robots or hard robots are effective as therapy robots. Resultantly, we confirmed that psychological stress relief effects obtained by interaction with soft robots are significantly higher than that with hard robots, particularly reducing tension and depression, improving vigor. And we also verified that physiological stress relief effects, especially increasing alpha waves, obtained by interaction with soft robots are significantly higher, too. In addition, we found that soft tactility may have effects of inhibiting reduction users' motivation for interaction. These results suggest that soft robots are more effective as therapy robots.