A diagnostic method for spatial distributions of D-D and D-3He fusion reactions has been developed. Refinement of collimation geometry and choice of a detector and a shielding foil resulted in a drastic improvement of signal separation from noise in collimated proton counting. The developed method was then applied and revealed proton yield distributions in an inertial-electrostatic confinement device running with a D2-3He mixture fuel gas. The result showed localized D-3He reactions on cathode gird surfaces. It also indicated considerable fractions of D-D reactions on anode grid and chamber wall surfaces as well as the cathode grid.