A reduction in the human arousal level can often leads to traffic or commondaily accidents. Hence, it is important to accurately detect the human arousal level. However, it is not easy to set up a method of monitoring this. The method introduced in this paper focusses on utilizing the skin potential level (SPL). The authors have developed and designed an ambulatory wristwatch type arousal level monitor based on the SPL. The monitor incorporates a new porous electrode with a dry conditioned surface, and adopts a new algorithm to detect relative changes in the SPL.