The Kyoto University Research Reactor Institute (KURRI) has a research program for constructing an experimental facility for an accelerator-driven subcritical reactor (ADSR) by using the Fixed Field Alternating Gradient (FFAG) accelerator. Some subcritical experiments were performed before constructing the FFAG by combining an existing critical assembly at the Kyoto University Critical Assembly (KUCA) with a Cockcroft-Walton-type accelerator. As one of the experiments, we examined the beam collimators for leading source neutrons to the core efficiently. From the experiment and design study, it was confirmed that the optimized collimator could introduce about 2.5 times the source neutrons. The collimator designed here will be adopted in future experiments in the KURRI research program for ADSR.