A sample in a gas state to be measured is forced to impinge on a structure in which a change in a characteristic including a mechanical deformation, an optical change, an electric change and a magnetic change is exerted due to the impingement of a gaseous sample, a gasified liquid sample or solid sample to cause at least one of changes in characteristics including a mechanical deformation, an optical change, an electric change and a magnetic change; and the molecular weight of the sample in a gas state is obtained.