This study analyzes melt progression with core degradation in the core and lower head of a boiling water reactor using a detailed simulation geometry of an in-house three-dimensional simulation code. Since the Fukushima Daiichi nuclear power plant accident of Tokyo Electric Power Company Holdings (TEPCO) in 2011, the condition of the lower head following melt progression has been a topic of interest worldwide. By taking advantage of the unique characteristics of the three-dimensional simulation code, we focus on achieving a more detailed simulation of the reactor pressure vessel in view of future Fukushima decommissioning processes. Molten corium forms in the core and progresses towards the lower head. When the corium reaches water remaining there, it is fragmented and forms fuel debris on the bottom of lower head. Hence evaluating molten corium breakup with water in the lower head is essential for the coolability of fuel debris and the integrity of the lower head to prevent the release of a large amount of radioactive isotopes. To simulate melt progression together with core degradation effectively, we focused on the development of the detailed simulation geometry comprising the core, the core support plate, and the lower head zones as well as the simplified pedestal of the primary containment vessel. The analysis capabilities due to the detailed simulation geometry were preliminarily evaluated under hypothetical conditions (i) melt progression from the core to the pedestal of primary containment vessel, (ii) failure of the core support plate zone due to molten corium, (iii) water pool behavior in the lower head, (iv) the relocation paths of molten materials via the control rod guide tube/control rod drive housing and instrument guide tube, and (v) mixing behavior of molten corium resulting from different fuel assemblies.