8-[2-(2-pentyl-cyclopropylmethyl)-cyclopropyl]-octanoic acid has a protein tyrosine phosphatase 1B (PTP1B) inhibitory action, an Akt activating action, a GSK-3² phosphorylation promoting action, and a suppressive action on Ä protein phosphorylation induced by A² 1-42 , and is useful as a therapeutic drug for Alzheimer-type dementia, an antidepressant and/or an anti-aging drug.