A shell assembly includes a shell including a depression and a flange, which go around a circumference of the shell, and the shell assemblies are arranged in N rows×M columns and fixed to a housing. The housing includes: N×M pieces of insertion holes into which respective ends of the shell assemblies are inserted; an engagement portion that is formed on an inner surface of each of the insertion holes on a position, which is closer to an obliquely-adjacent insertion hole of the insertion holes, and is engaged with the depression; and a slit that is formed by cutting the housing in a direction orthogonal to an extension direction of the insertion holes and includes a first groove and a second groove formed on an inner surface thereof.