8-[2-(2-pentyl-cyclopropylmethyl)-cyclopropyl]-octanoic acid has a protein tyrosine phosphatase 1B (PTP1B) inhibitory action, an Akt activating action, a GSK-3β phosphorylation promoting action, and a suppressive action on τ protein phosphorylation induced by Aβ1-42, and is useful as a therapeutic drug for Alzheimer-type dementia, an antidepressant and/or an anti-aging drug.