In order to get suitable ratio of the intervals between the adjacent transition regions in the recording medium in the black image area and white image area, distribution of magnetic force acting on the toners is analyzed using finite element method and proximate equations in longitudinal recording magnetography. Distance of 1/2λ between the adjoining transition regions in the recording medium was allocated to form one dot black image, while the distance of 3/2λ between the adjoining transition regions was allocated to form one dot white image, where λ is defined as regular size of one dot. The toner with the diameter of 10μm was employed. As pixel density of 400 dpi was employed, 1 dot corresponded to about 60μm. Then the distance of 1/2λ for a black dot corresponded to about 30μm and the distance of 3/2λ for a white dot corresponded to about 90μm. The following results are found. Toner is attracted strongly to the recording medium at the transition regions and that toner is attracted to the next toners each other at the intermediate region between the adjoining transition regions in the 1/2λ area. Then black image is formed around the 1/2λ area. On the contrary toners far from the transition regions get very weak attractive force. Then white image is formed around the 3/2λ area. However, the toners at the end of the 3/2 λ area get strong force from the recording medium and become a part of the black image. As a result it is considered that black image of λ and white image of λ are formed.