Disclosed is a polyester fiber which does not have disadvantages such as voids caused by catalyst particles or the like, exhibits a narrow range of physical properties variation, rarely undergoes the formation of fluffs. The polyester fiber comprises layered nanoparticles each of which comprises a bivalent metal and a phosphorus compound, is 5 to 100 nm on a side and has an interlayer spacing of 1 to 5 nm. Also disclosed are a tire code, a tire, a fiber material for reinforcing a belt, and a belt, each of which comprises the polyester fiber.