Objectives: To examine the psychological effects of forest therapy program on workers. Methods: The subjective symptoms index, a shortened version of the profile of mood states (POMS), and a semantic differential (SD) method were used to measure the psychological effects. Results: The evaluations were performed 3 days before, during, and 1, 3, and 5 days after the forest therapy. The following results were obtained: (1) the subjective symptoms improved before breakfast and continued for 5 days; (2) the mood evaluated using POMS improved before breakfast and continued for 3 days; and (3) “comfortable,” “relaxed,” and “natural” feelings evaluated using the SD method were enhanced before breakfast, lunch, and dinner during forest therapy. Conclusions: These results provided scientific evidence of the psychological effects of forest therapy program on workers.