Power spectral analysis of the heat-to-beat variations of heart rate (R-R interval) obtained using invasive and contact sensor has become widely used to quantify cardiac autonomic regulation, and also it leads to the estimation of a stress state. The aim of this study is to estimate a stress state from respiratory waves obtained by monitoring thorax and abdomen activity using 24 GHz wideband radar without making contact with the human body. This avoids additional preparations such as physical sensor attachment or special clothing, which can be useful for certain healthcare applications. If it is possible to estimate the stress state from heart and respiratory rate variability, we may be able to help in our stress management during sleeping, driving and working, which is nonrestrictive and noninvasive. In this paper, we found that it can estimate the stress state even from respiratory rate variability from the relationship between heart rate and respiration. We propose estimating technique of stress state using constant false alarm rate from respiratory rate variability and the effectiveness is also quantitatively evaluated by through experiment.