The property of light spot of zone plate in misfocusing conditions is considered. The effect of band width of illuminating light is discussed. Relative intensity distribution, radius of light spot and total energy in light spot have been calculated numericaly. Allowable limit of the largest focusing error of the zone plate which will be used in the soft-X-ray scanning microscope has been calculated. The limit of resolution of the soft X-ray scanning microscope has been shown.