A method of preparing a polyvinyl alcohol hydrogel contains the steps of charging an aqueous solution of a polyvinyl alcohol having a degree of hydrolysis of not less than 95 mol% and an average polymerization degree of not less than 500 into a container, the aqueous solution containing the polyvinyl alcohol in a concentration of not less than 5 wt.%, cooling the aqueous solution of the polyvinyl alcohol at a temperature of from - (minus) 9°C to - (minus) 3°C, subjecting the aqueous solution of the polyvinyl alcohol to physical stimulation, and then maintaining the aqueous solution of the polyvinyl alcohol at a temperature of from - (minus) 5°C to + (plus) 5°C. The polyvinyl alcohol hydrogel may be contained in a plastic container. An MRI phanton is composed of the plastic container containing such polyvinyl alcohol hydrogel.