Inside a first case outer frame portion as an outer frame of the first case, a plurality of first core pieces and a partition to separate a pair of adjacent first core pieces among the plurality of first core pieces are disposed. The first case has a shape capable of accommodating at least a part of the second core piece. The first case outer frame portion includes a first case accommodating portion as a portion of the first case outer frame portion that is capable of accommodating the plurality of first core pieces, and a first case cover portion to cover a space inside the first case accommodating portion.