Fuel rod bundles in a boiling water reactor (BWR) have complicated structures such as full-length fuel rods, part-length fuel rods, water rods, and spacers with swirl vanes. The boiling two-phase flow in the fuel rod bundle develops three-dimensionally, and these complicated flow channel structures affect the moderator density distribution, namely, the void fraction distribution. To improve the thermal margin and pressure loss of the fuel rod bundle, it is important to predict the three-dimensional phase distribution in the fuel rod bundle with high spatial resolution. In this study, we focus on the effect of spacers with swirl vanes on the boiling two-phase flow in a heated rod bundle. A boiling flow experiment was conducted using the test facility for 3D thermal hydraulics in light water reactors (SIRIUS-3D) and a linear accelerator-driven high-energy X-ray computed tomography (CT) system under the flow conditions, a system pressure of 7.2 MPa, an inlet mass flux of 1000 to 1500 kg/m²/s, and an inlet subcooling of 50 kJ/kg. The void redistribution behavior downstream of the spacer with swirl vanes was evaluated as an evolution of radial void fraction distribution in the subchannel.