A phantom (10, 20) for diagnosis by nuclear magnetic resonance imaging comprises a container made of a synthetic resin and a hydrogel sealingly contained in the container. The container has at least one opening (11, 12, 21) into which a sample material is inserted. The hydrogel is prepared by charging an aqueous solution of a polyvinyl alcohol into the container, cooling the aqueous solution to minus 10 DEG C or lower to obtain a cooled frozen mass and thawing the cooled frozen mass.
