A flat wiring material case includes a first case including an accommodation space, a holding portion, and a first engagement portion, and a second case including an accommodation space, a holding portion, and a second engagement portion, wherein the first case and the second case have an extension direction in which the flat wiring material extends, the first case and the second case are configured to be disposed at a first relative position and a second relative position, the first case and the second case are disposed side by side in a direction orthogonal to the extension direction at the first relative position, the first case and the second case are linearly disposed along the extension direction at the second relative position, and the first case and the second case are fixed at the second relative position.