Provided are effective and highly safe agents, medicaments and the like for ameliorating various disorders caused by brain hypofunction. Also provided is a preventing or ameliorating agent for brain hypofunction containing N-acetyl-D-mannosamine, a pharmaceutical composition for preventing, ameliorating or treating disorders due to brain hypofunction, containing an effective amount of N-acetyl-D-mannosamine and a pharmaceutically acceptable carrier, and a food comprising N-acetyl-D-mannosamine added thereto.