PURPOSE: To continuously produce a GI type synthetic resin light transmission by body having excellent optical performance, such as transmission loss and transmission band with a good productivity.

CONSTITUTION: This process for continuous production of the synthetic resin light transmission body as an optical fiber 23 having a continuous refractive index gradient from its peripheral part to its central part includes the following four stages: (I) A stage for continuously molding a hollow body by extruding a transparent thermoplastic polymer perpendicularly downward, (II) a stage for continuously packing a radical polymerizable monomer liquid 20 having affinity to this transparent thermoplastic polymer into the resulted perpendicular hollow body via a supply pipe 2, (III) a stage for continuously producing the solid body 21 by continuously radical polymerizing the packed radical polymerizable monomer liquid 20 without changing the position of the monomer liquid level and (IV) a stage for continuously heating and stretching the lowermost part of the resulted solid body 21.