PROBLEM TO BE SOLVED: To provide a drug which causes an enhancing action to a mitochondrial function in the body with high safety, a drug which reduces an oxidative stress, or a drug which causes an anti-aging action.SOLUTION: The invention provides a drug which causes an enhancing action to a mitochondrial function in the body, a drug which reduces an oxidative stress, or a drug which causes an anti-aging action, comprising as an active ingredient oligo lactic acids produced by heat condensation of L-lactic acids.