A multicore fiber includes: a first core configured to propagate an LP01 mode, an LP11 mode, and an LP21 mode light beam; and a second core configured to propagate an LP01 mode light beam, wherein a different mode interaction section is provided in which a propagation constant of the LP21 mode light beam propagated through the first core is matched with a propagation constant of the LP01 mode light beam propagated through the second core, a different mode non-interaction section is provided in which propagation constants of the LP mode light beams propagated through the first core are not matched with propagation constants of the LP mode light beams propagated through the second core, and the first core includes an inner core and an outer core surrounding the inner core with no gap and having a refractive index higher than a refractive index of the inner core.