To produce a potent antidementia $eta$-secretase inhibitor from a mushroom, the $eta$-secretase inhibitory activities of various mushroom extracts were determined. Methanol extracts of Lentinula edodes exhibited the highest inhibitory activity (40.1%). The inhibitor was maximally extracted when a fruiting body of L. edodes was treated with 50% methanol at 40$^{circ}C$ for 24 h.