The Super-Kamiokande Gadolinium (SK-Gd) project is the upgrade of the Super-Kamiokande (Super-K) detector via the introducing gadolinium sulfate.
For precise measurement in SK-Gd, detailed detector simulation is needed. Currently Super-K uses Geant3 based simulation, however, the development and the update of Geant3 was finished.. For the next 10 years Spuer-K operation, up-to-date detector simulation is needed. Especially, for diffuse supernova neutrino background search in SK-Gd, precise neutron simulation is crucial. Therefore, we are developing Geant4 based Super-K simulation software, named ’SKG4’. We started from the construction of Super-K geometry, and a comparison with the current Super-K detector simulation has been conducted. In this proceedings, current status of the development for SKG4 and the result of the several physics simulation will be reported.