A radiation focal position detecting method for detecting a positional displacement of a focal point of a radiation source in a radiation tomographic imaging apparatus is provided. The method includes providing a radiation absorber that covers parts of first and second detecting element regions, the parts lying on mutually adjoining sides of the first and second detecting element regions in a radiation detector including a plurality of detecting elements arranged in channel and slice directions, and specifying, based on intensities of radiation detected by the detecting elements in the first and second detecting element regions, a position of the focal point or an amount of movement of the focal point from a reference position.