A multicore polarization-maintaining fiber comprising: a plurality of cores; a cladding surrounding the plurality of cores; and a plurality of stress applying parts, wherein each core of the plurality of cores is sandwiched between two stress applying parts of the plurality of stress applying parts in a region surrounded by an outer circumferential surface of the cladding, wherein: a cross sectional area of each of the plurality of stress applying parts is greater than a cross sectional area of each of the cores; and the plurality of stress applying parts includes a first pair of stress applying parts and a second pair of stress applying parts, wherein the first pair of the stress applying parts sandwiches a first one of the cores therebetween and is disposed in a first direction to sandwich the first core in a cross section perpendicular to a longitudinal direction of the cladding and the second pair of the stress applying parts sandwiches a second one of the cores therebetween and is disposed in a second direction to sandwich the second core, the second direction being different from and intersecting with the first direction in the cross section, characterized in that the plurality of cores and the plurality of stress applying parts are disposed at positions in 90 degree rotational symmetry about the center of the cladding in the cross section. a plurality of cores; a cladding surrounding the plurality of cores; and a plurality of stress applying parts, wherein each core of the plurality of cores is sandwiched between two stress applying parts of the plurality of stress applying parts in a region surrounded by an outer circumferential surface of the cladding, wherein: a cross sectional area of each of the plurality of stress applying parts is greater than a cross sectional area of each of the cores; and the plurality of stress applying parts includes a first pair of stress applying parts and a second pair of stress applying parts, wherein the first pair of the stress applying parts sandwiches a first one of the cores therebetween and is disposed in a first direction to sandwich the first core in a cross section perpendicular to a longitudinal direction of the cladding and the second pair of the stress applying parts sandwiches a second one of the cores therebetween and is disposed in a second direction to sandwich the second core, the second direction being different from and intersecting with the first direction in the cross section, characterized in that the plurality of cores and the plurality of stress applying parts are disposed at positions in 90 degree rotational symmetry about the center of the cladding in the cross section. a cross sectional area of each of the plurality of stress applying parts is greater than a cross sectional area of each of the cores; and the plurality of stress applying parts includes a first pair of stress applying parts and a second pair of stress applying parts, wherein the first pair of the stress applying parts sandwiches a first one of the cores therebetween and is disposed in a first direction to sandwich the first core in a cross section perpendicular to a longitudinal direction of the cladding and the second pair of the stress applying parts sandwiches a second one of the cores therebetween and is disposed in a second direction to sandwich the second core, the second direction being different from and intersecting with the first direction in the cross section, characterized in that the plurality of cores and the plurality of stress applying parts are disposed at positions in 90 degree rotational symmetry about the center of the cladding in the cross section.