An object of the present invention is to provide a radioactive ray detector for enabling to reduce the parasitic capacity lower than that of the conventional art, which is generated between the semiconductor elements of the radioactive ray detectors neighboring with, and a radioactive ray detecting apparatus applying that therein. The radioactive ray detector, comprises a substrate, a first semiconductor element and a second semiconductor element, which are provided to face to each other with positioning the substrate therebetween, a first electrode pattern, which is electrically connected with the first semiconductor element on a surface facing to an opposite side of the substrate, and a second electrode pattern, which is electrically connected with the second semiconductor element on a surface facing to an opposite side of the substrate, wherein the first electrode pattern and the second electrode pattern are arranged not to overlap with each other, when seeing through the substrate in a direction of thickness thereof.