A resin composition for a primary resin layer of an optical fiber, the optical fiber comprising a glass fiber comprising a core and cladding, and the primary resin layer contacting with the glass fiber and coating the glass fiber, the resin composition comprising: a base resin containing a urethane (meth)acrylate oligomer, a monomer, a photopolymerization initiator, and a silane coupling agent; and surface-modified inorganic oxide particles having an ultraviolet curable functional group, which are inorganic oxide particles, into the surface of which an ultraviolet curable functional group is introduced by treating the surface of the inorganic oxide particles with a silane compound having an ultraviolet curable functional group, so that the surface-modified inorganic oxide particles are composed of an organic component derived from the ultraviolet curable functional group and an inorganic component, wherein the ultraviolet curable functional group is a methacryloyl group, and an amount of surface modification on the surface-modified inorganic oxide particles is 0.2 mg/m 2 or more and 2.8 mg/m 2 or less, wherein the amount of surface modification is calculated from the specific surface area of the surface-modified inorganic oxide particles and the ratio of the organic component, and is defined as follows: amount of surface modification (mg/m 2) = ratio of the organic component (% by mass)/specific surface area (m 2 /g) of the surface-modified inorganic oxide particles, wherein the ratio of the organic component, in % by mass of the organic component included in the surface-modified inorganic oxide particles, is determined by thermogravimetric/differential thermal analysis (TG/DTA) based on the weight change of the inorganic particles, and the specific surface area of the surface-modified inorganic oxide particles is measured by a nitrogen absorption BET method. a base resin containing a urethane (meth)acrylate oligomer, a monomer, a photopolymerization initiator, and a silane coupling agent; and surface-modified inorganic oxide particles having an ultraviolet curable functional group, which are inorganic oxide particles, into the surface of which an ultraviolet curable functional group is introduced by treating the surface of the inorganic oxide particles with a silane compound having an ultraviolet curable functional group, so that the surface-modified inorganic oxide particles are composed of an organic component derived from the ultraviolet curable functional group and an inorganic component, wherein the ultraviolet curable functional group is a methacryloyl group, and an amount of surface modification on the surface-modified inorganic oxide particles is 0.2 mg/m 2 or more and 2.8 mg/m 2 or less, wherein the amount of surface modification is calculated from the specific surface area of the surface-modified inorganic oxide particles and the ratio of the organic component, and is defined as follows: amount of surface modification (mg/m 2) = ratio of the organic component (% by mass)/specific surface area (m 2 /g) of the surface-modified inorganic oxide particles, wherein the ratio of the organic component, in % by mass of the organic component included in the surface-modified inorganic oxide particles, is determined by thermogravimetric/differential thermal analysis (TG/DTA) based on the weight change of the inorganic particles, and the specific surface area of the surface-modified inorganic oxide particles is measured by a nitrogen absorption BET method.