An optical fiber includes a core region and a cladding region. The cladding region includes a first cladding region on outer circumference of the core region, which includes a main-medium region and a sub-medium region having a refractive index lower than that of the main-medium region. The sub-medium region includes a plurality of inner sub-medium regions arranged along the outer circumference of the core region and a plurality of outer sub-medium regions arranged on outer of the inner sub-medium regions. The outer sub-medium regions have a lateral cross section larger than that of the inner sub-medium regions.