Carbon fibers achieve a high elongation percentage and a high tensile modulus of resin-impregnated strands while being lightweight. The carbon fibers are configured such that: the void content in a cross section of the fibers in the axial direction, the cross section including the long axis of a cross section of the fibers in the radial direction, is at least 0.3% by area, but not more than 5.0% by area; the average aspect ratio of the voids is at least 2.0, but not more than 50; and the average width of the voids in the cross section of the fibers in the radial direction is at least 3 nm, but not more than 100 nm.