Since objective biomarkers for major depressive disorder (MDD) are not readily available, clinical psychiatrists diagnose patients with MDD subjectively based on clinical interviews and diagnostic criteria. It often raises various concerns, including false responses by patients, subjective factors, and inexperience of the attendants leading to incorrect diagnosis. Here, we developed a self-monitoring system for simple and objective screening of MDD using a photoplethysmography (PPG) sensor and a 24-GHz microwave radar, which was based on the analysis of heart rate variability (HRV) during paced respiration and mental task conditions. In our previous study, we assessed the reactivity of HRV measurements during a mental task (random number generation) condition in patients with MDD and healthy control subjects. The HRV indices are less reactive in patients with MDD compared to healthy subjects during the mental task, which enabled us to identify the patients at risk for depression. In this study, the reactivity of HRV was measured not only in the mental task but also during paced respiration (i.e., 5-s inhalation and 5-s exhalation) conditions, thereby assessing more detailed autonomic nervous system (ANS) activity via HRV indices. To investigate the effect of paced respiration on MDD screening, we compared the ANS activity via HRV indices in with/without paced respiration conditions in 28 drug-naïve patients with MDD and 27 healthy control subjects. The result showed that ANS significantly activated during the paced respiration condition (p<;0.05). The sensitivity in detecting patients with MDD was 86% under paced respiration and mental task conditions, which was higher than the sensitivity (68%) under mental task condition alone.