An apparatus for X-ray photographing a tire (T, T1, T2), the apparatus comprising a rotary device for rotating a tire of any size chosen out of plural sizes with its center axis parallel with a given direction, a point-like X-ray source emitting X-ray in an angular range specified by a predetermined azimuth range (α) and a predetermined elevation angle range (β), and three linear line-sensors (3,4,5), each having a longitudinal direction and detecting one dimensional X-ray intensity distribution by means of light receiving elements aligned in the longitudinal direction, and producing a transmission X-ray image of the full circumference and full width of the tire, based on said one dimensional X-ray intensity distributions detected by said line-sensors for each angular position in tire rotation, - wherein said X-ray source is oriented so that said predetermined azimuth range includes a reference line within a plane perpendicular to the given direction, the reference line passing through the X-ray source and the center axis, - wherein said X-ray source is movable along said reference line according to the size of the tire mounted on the rotary device, so that said X-ray source can be placed at the nearest position to the inner surface of a tread portion of the tire as long as said predetermined elevation angle range covers an angular range including both bead cores with the tread portion in between, - wherein each of said three line-sensors is disposed so that at a plane perpendicular to a longitudinal direction of the respective line-sensor and passing through the X-ray source, the light receiving surface (3a, 4a, 5a) of the corresponding light receiving element is oriented perpendicular to a line headed to the X-ray source, - wherein a first line-sensor, being oriented parallel with said given direction, is placed within said predetermined azimuth range, in the radial outside of the tread portion of the tire mounted on the rotary device, - wherein second and third line-sensors, being disposed laterally outside of respective sidewall portions of the tire mounted on the rotary device with the respective longitudinal axes of the second and third line sensors parallel to the reference line and in a plane defined by the reference line and the given direction, and - wherein the second and third line-sensors are movable in the direction parallel with the center axis according to the size of the tire mounted on the rotary device, so that it can be as close as possible to respective sidewall portions of the tire; CHARACTERISED IN THAT: - the first line-sensor is placed at a position deviated from the plane defined by both said reference line and said center axis, having two degrees of freedom of motion within a plane perpendicular to said given direction, so that it can get as close as possible to the outer surface of the tread portion of the tire, according to the size of the tire mounted on the rotary device, keeping the orientation of said receiving surface perpendicular to the line headed to said X-ray source. - wherein said X-ray source is oriented so that said predetermined azimuth range includes a reference line within a plane perpendicular to the given direction, the reference line passing through the X-ray source and the center axis, - wherein said X-ray source is movable along said reference line according to the size of the tire mounted on the rotary device, so that said X-ray source can be placed at the nearest position to the inner surface of a tread portion of the tire as long as said predetermined elevation angle range covers an angular range including both bead cores with the tread portion in between, - wherein each of said three line-sensors is disposed so that at a plane perpendicular to a longitudinal direction of the respective line-sensor and passing through the X-ray source, the light receiving surface (3a, 4a, 5a) of the corresponding light receiving element is oriented perpendicular to a line headed to the X-ray source, - wherein a first line-sensor, being oriented parallel with said given direction, is placed within said predetermined azimuth range, in the radial outside of the tread portion of the tire mounted on the rotary device, - wherein second and third line-sensors, being disposed laterally outside of respective sidewall portions of the tire mounted on the rotary device with the respective longitudinal axes of the second and third line sensors parallel to the reference line and in a plane defined by the reference line and the given direction, and - wherein the second and third line-sensors are movable in the direction parallel with the center axis according to the size of the tire mounted on the rotary device, so that it can be as close as possible to respective sidewall portions of the tire; - the first line-sensor is placed at a position deviated from the plane defined by both said reference line and said center axis, having two degrees of freedom of motion within a plane perpendicular to said given direction, so that it can get as close as possible to the outer surface of the tread portion of the tire, according to the size of the tire mounted on the rotary device, keeping the orientation of said receiving surface perpendicular to the line headed to said X-ray source.