A radiation-image photographing apparatus comprises a radiation source, a radiation detector disposed in opposition to the radiation source for detecting radiation through an object to be examined and to generate an electrical signal proportional to the amount of incident radiation, a scanning device for changing the relative, positional relationship between the radiation source and the radiation detector, an analog-to-digital converter for converting the output signal from the radiation detector to a digital quantity, a memory for storing the digital signal, an arithmetic unit, and a display unit. A plurality of measurements of a two-dimensional radiation absorption distribution of the object disposed between the radiation source and the radiation detector is obtained while the relative positional relationship between the radiation source and the radiation detector is being changed, and a linear arithmetic operation is performed on the plurality of image measurements, or a set of data passing a point within the object to be photographed, thereby displaying a cross-sectional image on a given cross-section approximately parallel to the radiation detector plane within the object to be examined.