Abstract: To measure three-dimensional dose distributions with a dosimeter, such as a gel dosimeter. In an embodiment of the present invention, provided is a gel dosimeter having water, as solvent or disperse medium, clay particles that swell with water, and recording material precursor, which are dissolved or dispersed with each other. The recording material precursor has an atom or ion, wherein the atom or ion changes its valence number by reacting with both of a radical and a molecular radical derivative. The radical is to be generated from the water ionized by irradiation of radiation rays, and the molecular radical derivative is a molecule to be formed by bonding the radicals with each other. By the time radiation rays are irradiated, the gel dosimeter has been free from a substance identical to the molecular radical derivative and has lost fluidity.