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Recent psychophysiological studies have revealed a close relationship between mental states of human and secretion of hormones and immunological substances. In this study, we used a major secretory immune substance, secretory Immunoglobulin A (IgA), as a biomarker of stress caused by mental workload. In the experiment, subjects were required to conduct a simple mental workload repeatedly with intermediate break. The result shows that salivary IgA changed according to the work/break schedule in the time series. Moreover, an accumulative IgA change through out this period was observed. We proposed a simple mathematical model to explain this accumulative IgA change. Finally, the result of the experiment and simulation are discussed in relation to the possibility of estimation and/or prediction of subjective stress cause by mental workload; it implies the future development of the self-stress management system.