A radiation image detector with improved readout efficiency. The detector records a radiation image by generating and storing electric charges when irradiated with radiation, and the image is read out by irradiating readout light. The detector includes first line electrodes and second line electrodes disposed alternately with each other. An opaque line insulator that blocks the readout light is provided on each side face extending in the longitudinal direction of each second line electrode. The insulator has a width smaller than the distance between the first and second line electrodes. This arrangement allows the electric charges in the area of the photoconductive layer adjacent to the side faces of the first line electrodes to be fully discharged, and the electric charges in the area of the photoconductive layer adjacent to the side faces of the second line electrodes to be prevented from discharging.