Simultaneous measurements of neutrons and protons were carried out to identify D-D fusion reactions in an Inertial-Electrostatic Confinement Fusion (IECF) device, which is theoretically expected to produce D-D protons and neutrons in a dense plasma core at the center. Experimental results showed an excellent agreement of a measured proton energy with the predicted one, and a strong linear correlation between neutron and proton yields, both indicating conclusively D-D fusion reactions in the IECF device. It is also found, through comparison between neutron and collimated proton yields, that more than 98 % of the fusion reactions take place outside the central core region under the present experimental conditions.