A method is disclosed in which projection images of a certain cross-section of an object are produced through irradiation from various directions to the cross-section, and a three-dimensional image of the cross-section is reconstructed from the projection images. A convolution integral operation is performed on the projection images by means of a weighting function obtained from a function whose second differentiation is positive in at least a predetermined range of a spacial frequency domain and which has a positive first-differential coefficient at a point where the spacial frequency is equal to zero.