1. A radiation image capturing device comprising: a plurality of detector panels each including a collimator that aligns an incident direction of a radioactive ray and a detector that detects the radioactive ray, the incident direction of which is aligned by the collimator; a gantry mounted with the plurality of detector panels, the gantry rotating the mounted detector panels around a circumference, an object being substantially at a center of the circumference; a computer coupled to the gantry; and a tangential direction moving mechanism that moves, in a tangential direction of the rotational movement, the plurality of detector panels, wherein the computer is programmed to: calculate a first amount to move a first detector panel, of the plurality of detector panels, in the tangential direction based on a position where a size of a projection image of a region of interest of the object is a maximum in the tangential direction at a first rotational position and a length of the first detector panel in the tangential direction, calculate a second amount to move a second detector panel, of the plurality of detectors, in the tangential direction based on a position where a size of a projection image of the region of interest is a minimum in the tangential direction at a second rotational position and a length of the second detector panel in the tangential direction, wherein, when the gantry rotates the detector panels around the entire circumference and a projection image of the region of interest at each of a plurality of rotational positions is captured by the plurality of the detector panels, at least the first detector panel and the second detector panel are moved in the tangential direction by the calculated first amount and second amount, respectively, so that a size of a union of a capturing range of the projection image captured by the plurality of the detector panels corresponds to a size of the region of interest, and wherein the computer is further programmed to use the projection image captured by each of the plurality of the detector panels that have performed the rotational movement and the tangential direction movement to reconstruct a transaxial image of the region of interest.