By combining the Fixed Field Alternating Gradient (FFAG) -accelerator with the A-core of the Kyoto University Critical Assembly (KUCA), -accelerator-driven system (ADS) experiments are accomplished successfully using (p, n) reaction on the tungsten target which is located at core outside. In order to increase spallation neutron injection into this core, numerical analyses are performed by adapting the neutron or proton beam duct and the change of tungsten target position. When the tungsten target is moved into core closely, the neutron multiplication calculated by the ratio of neutrons by fission and supplied neutrons from external source increases gradually in inverse proportion to the distance between core and target. When this distance is changed from 70 to 20 cm, 430% reaction rate gain is observed relatively by comparing with 115In(n, n′)115mIn reaction. © Springer 2011.