Diagnosis for right-to-left shunt was determined by the assessment of shunt-rate, which was obtained by using 99 mTc-macroaggregated albumin. However, it is difficult to diagnose right-to-left shunt, using the normal level of shunt-rate measured by using conventional methods. To solve this problem, we investigated ourselves the normal level of shunt-rate. We researched 20 patients with pulmonary embolism, and they didn't have right-to-left shunt. We investigated three points for the normal level of shunt-rate. First, we examined the region of interest (ROI) area of the lungs to modify the upper level of gray scale. Second, we confirmed the difference between the whole visual field and body contour of the ROI area. Third, we examined the necessity whether we correct the background of whole body and the lungs. We resulted three points. First, stable right-to-left shunt rate is got to set that the upper level of gray scale is 35%. Second, there is no significant difference between the whole visual field and body contour of the ROI area. Third, correcting background isn't needed to get right-to-left shunt rate. The normal level of the shunt-rate was 12.6±2.8% in the condition. We are able to decide the optimal condition for the normal level of shunt-rate. It is important to evaluate the normal level of the shunt-rate fixed on each factor in each hospital.