A high-energy X-ray/gamma-ray imaging system based on a hydrogenated amorphous silicon(a-Si:H)/crystalline silicon(c-Si) heterojunction strip detector was developed. The imaging system will be applied in nondestructive testing of concrete structures. We fabricated 50-channel heterojunction strip detectors with a1mm pitch on 500μm thick p-type silicon wafers. The average leakage current was 2.9nA per channel at 120V reverse bias. Energy resolutions of 2.8keV FWHM at 59.5keV and 2.9keV FWHM at 122keV were obtained at 18℃. The position sensitivity of the strip detector was measured by edge-on irradiation with a 137Cs gamma-ray source. Edge-on gamma-ray imaging of a tungsten object using the prototype was performed. A module consisting of 20 stacked silicon strip detectors is being constructed.