In order to increase reproducibility of a reconstructed tomographic image without increasing the computational load, detection is performed by oversampling in (N+n) directions during imaging for detection by N detection elements. A vector having N×(N+n) elements is obtained, and vector decimation step is performed in which a total of n×N elements corresponding to a sequence {k} 30 denoting a decimation order are removed. In a discrete Radon transform step, a corresponding discrete Radon inverse matrix WSQ−140 is operated, and in an image data generation step, de-vectoring is performed, thereby tomographic image data are acquired. When oversampling is used, a discrete Radon inverse matrix WSQ−1 is obtained. Therefore, a tomographic image is obtained by matrix computation without resorting to iterative approximation.