This paper discusses the photoplethysmographic amplitude (PPGA) obtained from smartwatch. In recent years, the number of patients with mental illness due to mental stress has been increasing. Thereby, the evaluation of mental stress in daily life is necessary to prevent the mental illness. The conventional evaluation of mental stress uses biological information such as respiration, heart rate, saliva and photoplethysmography (PPG). Among them, PPG is measurable by a smartwatch daily. Therefore, we examined whether it is possible to evaluate the mental stress using PPG obtained by a smartwatch. In the mental stress evaluation, we focused on the PPGA, which is the characteristic point of PPG. This amplitude is able to evaluate the mental stress even with intermittent data. This is suitable for evaluation of smartwatches when the data may not obtain due to body movements. By contrast, the accuracy of acquiring PPGA of a smartwatch might be low due to various factors such as changes in blood flow or skin temperature. Hence, in this research, we proposed a correction method to obtain a stable and accurate PPGA. In the experiment, the effectiveness of this method examined by comparing the data of the photoplethysmograph and the corrected data of smartwatch after measuring with the photoplethysmograph and the smartwatch simultaneously.