An optical fiber connection structure which reduces MPI in the use of an optical fiber with a bend resistance improved by forming holes in the fiber, and a single-mode fiber which reduces MPI are provided. A second cladding portion of a second single-mode fiber 20 includes holes 28, and thus, the second single-mode fiber 20 has low bending loss. A portion of the second single-mode fiber 20 connected to a first single-mode fiber 10a is made solid by filling corresponding portions of the holes 28 over the length L0, and light in a mode LP11 is significantly attenuated in this portion, thereby reducing MPI.