As a new method that can contribute to 3D shape analysis, we propose Incremental Contour Flow (ICF), which divides a 3D object into segments separated by boundary surfaces at narrow parts. ICF utilizes a property of distance transform that generates local maxima in a center of swollen part and bottleneck-like structures in a narrow part. Core of segment is defined as a local maximum layer that has a higher value than surrounding layers in a layer structure generated by converting the distance values to integers. Voxels in a surrounding layer of a core are added to the core incrementally, thereby 3D voxels are grouped to as many segments as the cores. As shown in an example to be discussed, ICF can generates combined soap bubble-like boundary surfaces from a 3D object made from merged three spheres. Since human organs tend to be distinguished by its shape in medical 3D images, ICF can be useful in the medical imaging applications.