A method of measuring an analyte, comprising a step of measuring a detectable substance by using a reaction system including a formation reaction of the detectable substance based on a chemical reaction of the analyte contained in a sample, wherein a layered inorganic compound is caused to exist in the reaction system including the formation reaction of the detectable substance, whereby high-sensitivity measurement is made possible, the detectable substance can be stabilized to improve accuracy of the measurement, a rate of a chemical reaction is increased to enable quick measurement, and high-sensitivity measurement is made possible even in a reaction system which forms an insoluble substance. Also, it can be provided an analytical testing piece for measuring an analyte, by measuring a detectable substance by using a reaction system including a formation reaction of the detectable substance based on a chemical reaction of the analyte contained in a sample, wherein the testing piece comprises at least one test portion having a detection portion for detecting the detectable substance and contains a layered inorganic compound at least in the test portion, whereby diffusion and elution of a dyestuff or the like is prevented, more sensitive and accurate simple analysis is made possible, and easy handling is possible.