Preferably, the ionomer resin that is a copolymer of methacrylic acid or acrylic acid, and ethylene is a resin in which an acid content is 3 to 20 wt % and 20 to 80 wt % of the total acid is neutralized with lithium ions (Li+), natrium ions (Na+), zinc ions (Zn+), magnesium ions (Mg+) or the like.