A multicore fiber comprising: a cladding; and a plurality of core elements (10, 20) provided in the cladding, each of the plurality of core elements (10, 20) including a core (11, 21), an inner cladding layer (12, 22) that surrounds the core (11, 21), and a low-refractive index layer (13, 23) that surrounds the inner cladding layer (12, 22) and has a lower average refractive index than the cladding and the inner cladding layer (12, 22), the cores of all the core elements (10, 20) having the same diameter and a same refractive index, and the inner cladding layers (12, 22) of all the core elements (10, 20) having a same refractive index, wherein the plurality of core elements (10, 20) are arranged such that a specific core element is surrounded by three or more core elements, and the plurality of core elements (10, 20) includes a first core element having a first low-refractive index layer and a second core element having a second low-refractive index layer, the light confinement effect of the first low-refractive index layer of the first core element in its respective core is weaker than the light confinement effect of the second low-refractive index layer of the second core element in its respective core, the first low-refractive index layer of the first core element causes larger light confinement loss in its respective core than the second low-refractive index layer of the second core element does in its respective core, each low-refractive index layer (13, 23) is formed of a material having a lower refractive index than the cladding and the inner cladding layers (11, 22), and the first low-refractive index layer of the first core element is thinner than the second low-refractive index layer of the second core element. a cladding; and a plurality of core elements (10, 20) provided in the cladding, each of the plurality of core elements (10, 20) including a core (11, 21), an inner cladding layer (12, 22) that surrounds the core (11, 21), and a low-refractive index layer (13, 23) that surrounds the inner cladding layer (12, 22) and has a lower average refractive index than the cladding and the inner cladding layer (12, 22), the cores of all the core elements (10, 20) having the same diameter and a same refractive index, and the inner cladding layers (12, 22) of all the core elements (10, 20) having a same refractive index, wherein the plurality of core elements (10, 20) are arranged such that a specific core element is surrounded by three or more core elements, and the plurality of core elements (10, 20) includes a first core element having a first low-refractive index layer and a second core element having a second low-refractive index layer, the light confinement effect of the first low-refractive index layer of the first core element in its respective core is weaker than the light confinement effect of the second low-refractive index layer of the second core element in its respective core, the first low-refractive index layer of the first core element causes larger light confinement loss in its respective core than the second low-refractive index layer of the second core element does in its respective core, each low-refractive index layer (13, 23) is formed of a material having a lower refractive index than the cladding and the inner cladding layers (11, 22), and the first low-refractive index layer of the first core element is thinner than the second low-refractive index layer of the second core element.