A modified polyvinyl alcohol protected with a protecting group of the present invention is one wherein an amount of high-molecular weight body components of the modified polyvinyl alcohol having a weight-average molecular weight of 250,000 or more as determined by polyethylene glycol standards according to a gel permeation chromatography is 1000 ppm or less in the modified polyvinyl alcohol. The vertical space occupied by the cassettes is reduced allowing more cassettes to be stacked in a given space. The massage assembly include a pair of massage members.