An imaging apparatus comprising: a detector including a plurality of pixels arranged in an array, wherein the pixel includes a conversion element for converting a radiation or a light into an electric charge, and the detector performs an imaging operation for outputting an image data according to an irradiation with the radiation or the light; and a controlling unit for controlling an operation of the detector including the imaging operation, wherein the imaging operation includes a first imaging operation in which the detector is configured to output an image data according to the irradiation with the radiation or the light in a first irradiation field of the detector corresponding to a part of the plurality of pixels, and a second imaging operation in which the detector is configured to output an image data according to the irradiation with the radiation or the light in a second irradiation field of the detector of an area larger than that of the first irradiation field, the controlling unit is configured to control the operation of the detector such that, responsive to a changing from the irradiation in the first irradiation field to the irradiation in the second irradiation field, the detector performs an initializing operation during a period between the first and second imaging operations for initializing the conversion element, and the second imaging operation includes an accumulating operation in which the detector is configured to convert the radiation or the light irradiated onto the conversion element into the electric charge, an image output operation in which the detector is configured to output the image data based on the electric charge, and an initialization operation performed before the accumulating operation, characterized in that the controlling unit is configured to control the detector such that the initializing operation (K2, K2') during the period between the first and second imaging operations and the initialization operation (K2, K2') of the second imaging operation are performed during the same length of a period. a detector including a plurality of pixels arranged in an array, wherein the pixel includes a conversion element for converting a radiation or a light into an electric charge, and the detector performs an imaging operation for outputting an image data according to an irradiation with the radiation or the light; and a controlling unit for controlling an operation of the detector including the imaging operation, wherein the imaging operation includes a first imaging operation in which the detector is configured to output an image data according to the irradiation with the radiation or the light in a first irradiation field of the detector corresponding to a part of the plurality of pixels, and a second imaging operation in which the detector is configured to output an image data according to the irradiation with the radiation or the light in a second irradiation field of the detector of an area larger than that of the first irradiation field, the controlling unit is configured to control the operation of the detector such that, responsive to a changing from the irradiation in the first irradiation field to the irradiation in the second irradiation field, the detector performs an initializing operation during a period between the first and second imaging operations for initializing the conversion element, and the second imaging operation includes an accumulating operation in which the detector is configured to convert the radiation or the light irradiated onto the conversion element into the electric charge, an image output operation in which the detector is configured to output the image data based on the electric charge, and an initialization operation performed before the accumulating operation, characterized in that the controlling unit is configured to control the detector such that the initializing operation (K2, K2') during the period between the first and second imaging operations and the initialization operation (K2, K2') of the second imaging operation are performed during the same length of a period.