To provide a weed-proof joint filler capable of stably suppressing propagation of weeds over a long period of time, reducing operator burdens during work, and allowing the work to be continued as it is even when a construction object is wet.

A weed-proof joint filler 1 mainly includes subtle granular silica sand 4 whose particle size is adjusted, a PP resin 5 which has a particle size larger than the silica sand 4 and is formed in a granular shape, and castor oil 6. Then, three kinds of components are mixed to be uniformly dispersed, and an obtained clay-like mixture is extrusion-molded so as to be a long bar-like body of a circular cross section by utilizing an extrusion molding machine. Also, when constructing the weed-proof joint filler 1 in a boundary face gap W, after the weed-proof joint filler 1 is mounted on the boundary face gap W, pressurized and closely attached to the space of the boundary face gap W, the weed-proof joint filler 1 is ignited by bringing a flame F close, and combustible components contained in the weed-proof joint filler 1 are burned.