A series of accelerator-driven system (ADS) experiments were carried out by supplying spallation neutrons to a subcritical core through the injection of 100MeV protons onto a tungsten target of 80mm diameter and 10mm thickness. The proton beams from the fixed field alternating gradient (FFAG) accelerator were generated at 30 Hz repetition rate and 10 pA current. Neutron detectors were set at three positions to obtain the information on the detector position dependence of the prompt neutron decay measurement. The prompt and delayed neutron behaviors, which were an exponential decay behavior and a slowly decreasing behavior, respectively, were experimentally confirmed by observing the time evolution of neutron density in ADS. The results show that subcriticality was deduced from the prompt neutron decay constant by the extrapolated area ratio method. The calculated reaction rate distribution approximately agreed with the experimental results within the statistical errors in the experiments.