A back projection method in CT image reconstruction method in which projection data obtained by irradiating a subject with electromagnetic wave and detecting the electromagnetic wave transmitted through the subject are back-projected onto a CT image reconstruction region virtually set on a region of interest of the subject, said back projection method comprising: generating a projected image redundant arrangement form in which the projection data are arranged so as to be redundant and gathered to access units; and performing a vector operation for the projection data arranged in the projected image redundant arrangement form. According to the back projection method of the invention, the vector operation can be performed efficiently while real-number arithmetic operation which is necessary for addressing can be omitted. Accordingly, back projection calculation can be performed speedily.