The preparation of 4-([6-(methacryloyloxy)hexyl]oxy)benzenecarboxylic acid is as follows:6-(4-Formylphenoxy)hexyl methacrylate in an amount of 116.14 g, 2100 mL of methanol, an aqueous sodium dihydrogen phosphate solution (obtained by dissolving 43 g of sodium dihydrogen phosphate dihydrate in 570 mL of water) and 73 mL of a 30% hydrogen peroxide solution were added one by one. An aqueous sodium chlorite solution (obtained by dissolving 61.9 g of sodium chlorite with the purity of 80% in 500 mL of water) was added by dropping. After dropping, the reaction liquid was stirred at 45° C. for 3 hours, and the reaction was finished. After slowly cooling the reaction liquid to 20° C., 2250 mL of water was dropped to the reaction liquid, and a solid was precipitated. The solid was collected by filtration, and 72.7 g of 4-((6-(methacryloyloxy)hexyl)oxy)benzoic acid was thus obtained.