There is provided an agent having at least one of an improving effect on vascular endothelial functions and an inhibitory effect on vascular intimal thickening, as well as a prophylactic of arteriosclerosis, which have excellent safety, improve functions associated with the vascular endothelium, have effects of preventing various diseases associated with vascular endothelial functions and of inhibiting vascular intimal thickening, and may be expected to provide prophylactic effect on arteriosclerosis or the like. The agents of the present invention contain, as an active component: (a) Xaa-Pro-Pro, (b) a hydrolysate of animal milk casein containing Xaa-Pro-Pro, or a concentrate thereof, or (c) a fermentation product containing Ile-Pro-Pro and/or Val-Pro-Pro obtained by fermenting a starting material containing milk protein with a bacterial strain of the species Lactobacillus helveticus.