To provide an image processing device and method capable of achieving a high-speed reconstruction process to a cone-beam CT image, an area necessary for reconstruction in a projection image is determined from a slice position to be reconstructed, and a convolution process is executed by using the determined area. Moreover, the slice position to be reconstructed is divided into plural blocks, the areas of the projection image corresponding thereto are cut out, and the processing unit is set for each block as local data of each block, and the set unit is allocated with respect to each operation, whereby parallel processes are executed.