An image reconstruction processing method according to this invention is characterized by adding weight to conventional reconstruction processing steps. That is, weighting is performed when performing the reconstruction process based on optimization calculations of a multivariate function in which a digital image is unknown, and which consists of data function generalized from the likelihood function of Poisson distribution. When a weight coefficient is a constant independent of element data, for example, the weight coefficient is set based on directionality of linear noises occurring in a reconstruction image, or the weight coefficient is set based on detection depth position information of detector elements of DOI detectors (step S1). By weighting partial functions with weight coefficients of back projection to a reconstruction image of the element data corresponding to the partial functions, artifacts appearing on the image can be suppressed, or the spatial resolution of the image can be improved.