12-molybdophosphoric acid encaged in the supercages of Y-type zeolite was synthesized from molybdenum oxide and phosphoric acid, in a slurry mixture of Y-type zeolite crystals and deionized water. After thorough washing in hot water, the 12-molybdophosphoric acid was found to remain in the Y-type zeolite, at an amount estimated to be 0.09 g (g-support)(-1). Catalysts thus obtained were found to show activity for the esterification of acetic acid with ethanol, indicating that this catalyst could be used as a solid acid catalyst in various liquid phase reactions which involve water.