In this study, we aimed to develop a simple and non-invasive method to estimate the cross-sectional area of abdominal muscles using images from an ultrasound imaging diagnostic device. The target sites were the rectus abdominis muscle, abdominal external oblique muscle, and abdominal internal oblique muscle. The subcutaneous tissues were imaged with the B mode and stored. Imaging sites were from the umbilical region to a point that intersected the line drawn straight down from the axilla (the edge of the external and internal oblique muscles). We obtained images by shifting the width of the probe. Using the analytical software developed in this study, we input the obtained images, adjusted the position for the images to line up, and identified the area of each muscle group and calculated the cross-sectional area. We hope that the analytical software developed in this study will be used as a simple and non-invasive method to estimate the cross-sectional area of muscles, regardless of the types of ultrasound imaging diagnostic devices.