1. A spectral type radiation imaging system in which x-ray imaging is performed using a contrast agent, comprising: an x-ray source for emitting a quasi-monochromatic x-ray beam to be projected to an object, the energy bandwidth of which extends between, above and below the absorption edge energy of an element constituting the contrast agent, wherein said x-ray source comprises as asymmetrically cut crystal for asymmetrically reflecting an incident x-ray beam to provide the quasi-monochromatic x-ray beam; a first x-ray detector for detecting x-rays penetrating through the object, said first x-ray detector comprising a scintillator made of fluorscent material containing the contrast-agent constituting element; and a second x-ray detector, at a location behind the first x-ray detector so that the first x-ray detector is interposed between the x-ray source and second x-ray detector, for detecting x-rays penetrating through said first x-ray detector.