A method for obtaining information on a mass of an object by time-of-flight mass spectrometry. This method includes placing colloidal metal particles for promoting ionization of the object inside the object at a depth ranging from 0.1 nm to 100 nm in opposition to a primary beam for the ionization; irradiating the object with the primary beam selected from the group of ions, neutral particles, and electrons, which can be focused, pulsed, and are capable of scanning, and laser beams, which can be focused, pulsed, and are capable of scanning to ionize a constituent of the object and to allow the ionized constituent to fly out of the object; and obtaining information on the mass of the flying constituent of the object by time-of-flight mass spectrometry.