[en] We have developed a novel low-cost gamma-ray imaging Compton camera γI that has a high detection efficiency. Our motivation for the development of this detector was to measure the arrival directions of gamma rays produced by radioactive nuclides that were released by the Fukushima Daiichi nuclear power plant accident in 2011. The detector comprises two arrays of inorganic scintillation detectors, which act as a scatterer and an absorber. Each array has eight scintillation detectors, each comprising a large CsI (Tl) scintillator cube of side 3.5 cm, which is inexpensive and has a good energy resolution. Energies deposited by the Compton scattered electrons and subsequent photoelectric absorption, measured by each scintillation counter, are used for image reconstruction. The angular resolution was found to be 3.5° after using an image-sharpening technique. With this angular resolution, we can resolve a 1 m"2 radiation hot spot that is located at a distance of 10 m from the detector with a wide field of view of 1 sr. Moreover, the detection efficiency 0.68 cps/MBq at 1 m for 662 keV (7.6 cps/μSv/h) is sufficient for measuring low-level contamination (i.e., less than 1 μSv/h) corresponding to typical values in large areas of eastern Japan. In addition to the laboratory tests, the imaging capability of our detector was verified in various regions with dose rates less than 1 μSv/h (e.g., Fukushima city).