In a time-of-flight type, secondary ionization mass-spectrometry apparatus, spread of a primary beam irradiating a sample is calculated through a numeric or practical-experiment process, and the spread is used as a blurring function to restore an image to reduce blur. Provided is an image processing method of a mass-spectrum image derived by irradiating a surface of a sample with a converged short pulsed primary beam, while changing an irradiating position, to display two-dimensionally a signal intensity based on a ratio of mass to an electric charge relating to a mass spectrum, derived by a time-of-flight secondary ion mass spectrometer. The method includes the steps of: calculating a blurring function based on a shape of the primary beam incident on the surface of the sample; and restoring the mass-spectrum image based on the blurring function.