Recent psycho-physiological studies have revealed close relationships between human mental stresses and secretion of hormones. However, a precise elaboration of fluctuations in the secretion of these biomarkers in the time series against stress; especially against a rather mild stressful task is not yet clearly illuminated. In this study, we used the dehydroepiandrosterone (DHEA), a major steroid hormone, as a biomarker of mental stress for 1) illustrating the precise stress-response in the time series, and 2) investigating a congruity of DHEA as the stress-biomarker. In the experiment, subjects were inscribed to conduct a simple, easy, and monotonous mental arithmetic task for about an hour with intermissions. As a result, salivary DHEA concentration depicted an accumulative increment over the experiment period, while no marked difference was obtained in the heart rate and its variability. It suggests the slow and long-lasting properties in the stress-response of DHEA unlike as in autonomous nervous system indices, and therefore plausibly demonstrates the possible candidacy of DHEA as a biomarker for a mild stressor.