Heating implement for alleviation of stiffness

The purpose of the present invention is to provide a novel means that makes it possible to more easily alleviate stiffness caused by rheumatoid arthritis or osteoarthritis. Another purpose of the present invention is to provide a skin heat retention method. Provided is a heat-generating implement for alleviation of stiffness caused by rheumatoid arthritis or osteoarthritis, said heat-generating implement comprising a heat-generating composition that generates heat upon contact with oxygen and an accommodation bag that accommodates the heat-generating composition, wherein at least part of the accommodation bag is constituted by an air-permeable packaging material, and the temperature of heat generation is not higher than 47 DEG C. Also provided is a skin heat retention method comprising a step for setting a heat-generating implement onto part of the skin of a subject having stiffness caused by rheumatoid arthritis or osteoarthritis and warming said part of the skin to 37-42 DEG C for not less than four hours.