1. A method of measuring in-plane distribution of a target object contained in a sample that is dispersedly placed on a substrate, which comprises the steps of: treating the sample to promote ionization of the target object; determining a mass and flying amount of an ion containing the target object or a component of the target object by irradiating an ion beam to the sample and performing time-of-flight secondary ion mass spectrometry of the ion that flies from a portion in the sample where the ion beam is irradiated; and determining the in-plane distribution of the target object from measurement data obtained by the step of determining the mass and flying amount of the ion at plural portions by scanning the beam on the sample plane, wherein the step of treating the sample to promote ionization of the target object comprises a step of contacting an aqueous solution of an acid that does not crystallize at an ordinary temperature with the sample.