The collective cell migration is thought to be a dynamic and interactive behavior of cell cohorts which is essential for diverse physiological developments in living organisms. Recent studies revealed that topographical properties of the environment regulate the migration modes of cell cohorts, such as diffusion versus contraction relaxation transport and the appearance of vortices in larger available space. However, conventional in vitro assays fail to observe the change in cells behavior in response to the structural changes. Here, we have developed a method to fabricate the flexible three-dimensional structures of capillary microtunnels to examine the behavior of vascular endothelial cells (ECs). The microtunnels with altering diameters were formed inside gelatin-gel by spot heating a portion of gelatin by irradiating the µm-sized absorption at the tip of the microneedle with a focused permeable 1064 nm infrared laser. The ECs moved and spread two-dimensionally on the inner surface of capillary microtunnels as monolayer instead of filling the capillary. In contrast to the 3D straight topographical constraint, which exhibited width dependence migration velocity, leading ECs altered its migration velocity accordingly to the change in supply of the cells behind the leading ECs, caused by the progression through the diameter altering structure. Our findings provide insights into the collective migration properties in 3D confinement structures as fluid-like behavior with conservation of cell numbers.