An object of the present invention is to provide a carbon fiber which exhibits excellent strength development rate when used in a composite material. The present invention that solves the problems is a carbon fiber which simultaneously satisfies the following formulae (1) and (2): Lc/d≤3  (1) TS×d×Lc>6.0×105  (2) wherein: Lc is an X-ray crystallite size (Å), d is a filament diameter (μm), and TS is a strand tensile strength (MPa).