Provided is a therapeutic agent for Alzheimer's disease. The therapeutic agent combines: (I) (S)-7-(2-methoxy-3,5-dimethylpyridin-4-yl)-1-(tetrahydrofuran-3-yl)-1H-pyrazolo[4,3-c]quinolin-4(5H)-one, which is represented by formula (I), or a pharmaceutically acceptable salt thereof; and (II) memantine, which is represented by formula (II), or a pharmaceutically acceptable salt thereof.