Exposure index (EI) is expected as important standard for the optimization of image quality and exposed dose in digital radiography. It is set a region of interest (ROI) in a radiograph, to obtain a histogram of pixel values within an ROI, it is computed as EI of the image representative value of the histogram. In this study, we examined the effects of setting the ROI for EI computing. We set five different ROIs in chest or cervical spine frontal radiograph, and compared the computed EI in each ROIs. From the result, in some cases, we found that EI varied greatly as those compared to the case of ROI set in full image. We should consider about setting the ROI for EI computing when we use EI as standard for optimization of image quality and exposed dose in digital radiography.