A radiation detector comprising: a conversion unit that comprises a plurality of pixels having a plurality of first conversion elements (101, 101') for outputting image information in accordance with an intensity of radiation that enters the conversion unit; a second conversion element which is laid out in the conversion unit; and wherein the plurality of pixels comprises a first pixel which has a first conversion element of the plurality of first conversion elements and has no second conversion element, and a second pixel which has a first conversion element of the plurality of first conversion elements and the second conversion element, and the first conversion element of the second pixel has an area smaller than that of the first conversion element of the first pixel such that the plurality of pixels are arranged at equal pitches, a processing circuit unit which is electrically connected to the second conversion element and which is adapted to obtain at least one of a total dose of radiation, a timing of a start of an irradiation of radiation, and a timing of a stop of an irradiation of radiation that enters the conversion unit, and characterized in that said processing circuit unit comprises: addition means for adding a charge output from said second conversion element; integral means for integrating the charge added by said addition means; comparison means for comparing an integral value obtained by said integral means with a threshold value which is set in advance; and radiation stop means for, when said comparison means determines that the integral value is larger than the threshold value, stopping radiation with which the conversion unit is irradiated. a conversion unit that comprises a plurality of pixels having a plurality of first conversion elements (101, 101') for outputting image information in accordance with an intensity of radiation that enters the conversion unit; a second conversion element which is laid out in the conversion unit; and wherein the plurality of pixels comprises a first pixel which has a first conversion element of the plurality of first conversion elements and has no second conversion element, and a second pixel which has a first conversion element of the plurality of first conversion elements and the second conversion element, and the first conversion element of the second pixel has an area smaller than that of the first conversion element of the first pixel such that the plurality of pixels are arranged at equal pitches, a processing circuit unit which is electrically connected to the second conversion element and which is adapted to obtain at least one of a total dose of radiation, a timing of a start of an irradiation of radiation, and a timing of a stop of an irradiation of radiation that enters the conversion unit, and characterized in that said processing circuit unit comprises: addition means for adding a charge output from said second conversion element; integral means for integrating the charge added by said addition means; comparison means for comparing an integral value obtained by said integral means with a threshold value which is set in advance; and radiation stop means for, when said comparison means determines that the integral value is larger than the threshold value, stopping radiation with which the conversion unit is irradiated. addition means for adding a charge output from said second conversion element; integral means for integrating the charge added by said addition means; comparison means for comparing an integral value obtained by said integral means with a threshold value which is set in advance; and radiation stop means for, when said comparison means determines that the integral value is larger than the threshold value, stopping radiation with which the conversion unit is irradiated.