Provided are a size adjuster and a headwear using the same having advantages of increasing exterior aesthetical features and automatically adjusting a circumference of a wearing portion of a headwear. The size adjuster may include: a first guide having one end connected to the wearing portion; a second guide having one end connected to the wearing portion corresponding to a position of the first guide; a case having an inner portion connected to the respective other ends of the first and second guides and where the first and second guides move through; and an elastic band extended or contracted by connecting to the first and second guides.