There is provided a therapeutic agent for Alzheimer's disease for combined use of (S)-7-(2-methoxy-3,5-dimethylpyridin-4-yl)-1-(tetrahydrofuran-3-yl)-1H-pyrazolo[4,3-c]qui noline-4(5H)-one represented by formula (I):or a pharmaceutically acceptable salt thereof, and memantine represented by formula (II):or a pharmaceutically acceptable salt thereof.