Three-dimensional data visualization employing the CT and MRI techniques is widely used to understand the complex internal structure of the human body. However, the position and depth information often become unclear when three-dimensional data are rendered transparently. In this study, we aimed to improve the accuracy of the perceived 3D structure by introducing multiple iso-surfaces as a visual guide. For the purpose, we conducted psychophysical experiments. The experimental results suggest that multiple isosurfaces improve the accuracy of the perceived depth. It was also found that the accuracy of perceived depth changes with the distance between the iso-surfaces.