In order to estimate human stress levels, it is necessary to use nonivasively measured biological information. The purpose of this paper is to evaluate whether the salivary amylase activity is a usable indicator of stress. The salivary amylase activity was compared with other standard methods for the noninvasive evaluation of stress, such as heart rate and cortisol concentration in saliva. Fifteen normally healthy subjects were made to pedal an ergometer as a means to induce physical stress, under constant load conditions, following which their heart rates were measured.Before and during this workload, saliva samples were collected in order to measure the salivary amylase activity and the cortisol concentration. It was found that there is a positive correlation between the heart rate and the physical stress. Also, the changes in salivary amylase activity showed a similarity with changes in the heart rate caused by the workload. Subsequently, the salivary amylase activity of five subjects was measured over a month period. The results indicated that for each individual there may exist a stable level for the salivary amylase activity. The studies suggest that the monitoring of salivary amylase activity could be used as an efficient index to noninvasively measure the stress.