Disclosed is a levosimendan compound, an optically active isomer thereof, or a pharmaceutically acceptable salt thereof, which is used for the prevention and/or treatment of diseases caused by tau aggregation or phosphorylation, such as neurodegenerative diseases (e.g., Alzheimer disease). In accordance with yet another aspect of the present invention, there is provided a health functional food for preventing or treating tau aggregation-related diseases, containing a levosimendan compound or a pharmaceutically acceptable salt, stereoisomer or solvate thereof.