Multicore optical fiber and method for producing multicore optical fiber

A multicore optical fiber which is provided with: a core that is configured from quartz glass doped with at least fluorine and has a relative refractive index difference of from -0.30% to -0.10% based on the refractive index of quartz glass; and a cladding that is configured from quartz glass doped with at least fluorine and comprises a first cladding covering the outer circumference of the core and a second cladding provided on the outside of the first cladding. The relative refractive index difference between the first cladding and the core is 0.8% or more. The refractive index of the second cladding is higher than the refractive index of the first cladding, but lower than the refractive index of the core.