To provide a side board having a returnable packaging material.

The side board includes a body wall composing one side face of the packaging material, and rib walls folded from ends of the body wall. The body wall has a folding portion allowing the body wall to be folded vertically. Each rib wall is divided into upper and lower splits through a cut line continuous with the folding line of the body wall, and has a fitting portion consisting of a recession formed on one of the upper and lower splits and a projection formed on the other of the splits. The rib wall can restrain horizontal move when the splits are fitted together, and can be folded vertically when the splits are released from mutual engagement.