The object is to find a nitric oxide synthase activator, a method for the administration of the activator, and the amount of the activator to be administered. Disclosed is a nitric oxide synthase activator comprising a midkine family protein or a midkine derivative as an active ingredient. Specifically disclosed is a nitric oxide synthase activator which is intended to be administered through the blood, a coronary artery or a vein and which comprises a midkine family protein or a midkine derivative as an active ingredient.
