A core part of a multimode optical fiber including the core part and a cladding part has a structure composed of a plurality of concentric layers in which a refractive index is decreased stepwise from a first core layer as an innermost layer to a third core layer as an outermost layer. The structure having the plurality of layers is formed by adjusting a quantity of addition of fluorine to silica glass. Fluorine is added to the cladding part so that a refractive index is lower than that of the third core layer as the outermost layer of the core part.