The purpose of this chapter is to clarify the physiological relaxation effect of a forest environment using field tests. We conducted field experiments in 35 forests across Japan. The subjects were twelve male university students at each location (420 in total; age range 21.8 ± 1.6 years). On the first day, 6 subjects were sent to a forest area, and the others to a city area. On the second day, each group was sent to the other area. The subjects walked (for 16 ± 5 min) around their assigned areas, and sat on chairs viewing the landscapes of their assigned areas (for 14 ± 2 min). Salivary cortisol, blood pressure, pulse rate, and heart rate variability (HRV) were used as indices. Measurements were taken at the place of accommodation in the morning, before and after walking, and before and after viewing at their assigned field areas. The R-R interval was also measured during the walking and viewing periods. The results show that forest environments promote lower concentrations of cortisol, lower pulse rate, lower blood pressure, greater parasympathetic nerve activity, and lower sympathetic nerve activity than city environments. These results will contribute to the development of a research field dedicated to forest-based therapy.