A GI type optical fiber of the present invention is a GI type optical fiber having a core component and a cladding component disposed around the outer periphery of the core component, the core component includes a polymer containing at least 55 wt % chlorostyrene monomer and a dopant, and the cladding component includes a polymer of a monomer containing at least 35 wt % methyl methacrylate. It is an object of the present invention to provide a GI type optical fiber in which chlorostyrene is used as the predominant component of the monomer that constitutes the core component, and therefore has excellent transparency and good flexibility, and allows high-speed communication.