To prevent a joint filler from coming off from a back split groove, even when the back split grove is expanded or shrunk due to a change in the temperature or the humidity of an atmosphere and secure safety for a long time by fitting the joint filler into the back split groove in an elastically contact state.

This joint filler 10 made of an elastic material is fitted into a back split groove 21 formed to a depth reaching almost the center part of a wood 20 in the longitudinal direction. Further, a leg part 12 with a smaller section than the width of the back split groove 21 is formed protruding toward the inner end of the back split groove 21 from the inner face of a surface layer part 11 which is elastically set into contact with the outer end part of the back split groove 21. In addition, a lip part 13 which is set into contact elastically with the lateral wall 22 of the back split groove 21 is formed protruding from both sides of the leg part 12.