This study aimed to clarify the effects of watching forest videos on prefrontal cortex activity. We examined oxy-hemoglobin (oxy-Hb) levels in the prefrontal cortex using near infrared spectroscopy (NIRS) and measured the changes in relaxation level using the rating scale of emotion as defined in terms of relaxation (RE scale), and the free description data after watching the videos. We recruited 21 healthy female university students for this study. All subjects watched a forest video and an urban video in a random order; no significant differences were found in comparisons between the two conditions during and after watching the videos. Comparisons of oxy-Hb levels in the prefrontal cortex during and after watching the two videos showed a significant increase after watching the forest video. Significantly decreased scores of three RE scale items, i.e., “anxiety-relief,” “restrictive-free,” and “discomfort-comfort,” were observed with the urban video. The forest video significantly improved scores of the following items: “feeling high-stable,” “tense-relaxing,” “anxiety-relief,” “restrictive-free,” and “discomfort-comfort.” These results suggest that the forest video activates the left prefrontal cortex and subjectively enhances a relaxed feeling, after watching.