In the improved neutron image detector, MgB₂ enriched in a constituent element ¹⁰B of wide energy gap is used as a neutron detection plate, which is provided at the center and the four corners with a phonon sensor comprising an insulation layer overlaid with Mg¹¹B₂ enriched in ¹¹B of narrow energy gap in order to detect phonons resulting from the generation of -rays which occurs in the detection plate upon incidence of neutrons, and sensor's signal intensity and signal propagation time are used to detect the incident position of neutrons.