An attachment tool has a plurality of insertion holes. An optical adapter is fitted into each of the insertion holes. At this time, a holding structure for the optical adapter and the attachment tool has a swaying mechanism that allows the optical adapter to sway against the attachment tool. When a force greater than a prescribed strength is applied to any one of the optical adapters, the swaying mechanism allows the optical adapter to sway in a direction in which the optical adapters are provided side by side. When the force is offloaded, the swaying mechanism can hold the optical adapter in a direction substantially vertical to the attachment tool.