To provide the method and device for manufacture of a gradient index plastic optical fiber which are superior in operability and can suppress a large-sized scale of facilities with the increase of the production, easily give a specific refractive index distribution to a plastic optical fiber, and stably and successively manufacture a gradient index plastic optical fiber of stable quality with arbitrary length.

A fiber material obtained by blending a unpolymerized compound having a higher refractive index than a polymer as a base with the polymer is spinned by the fiber manufacturing device 1 to continuously manufacture a fiber 5, which is dipped in diffusion tanks T1 to Tn in order from the monomer 17 of the lowest viscosity in the tank to diffuse the unpolymerized compound and monomer 17 in the fiber 5, thereby forming a refractive index distribution. Then the fiber 5 has the monomer 17, diffused in itself, polymertized and caked by a drying and heating device 15 and is passed through a drawing device 19 and a clad forming device 21 and then wound by a winding machine 25.