PURPOSE: To contrive an improvement in bonding strength of an dowel hole, by making molten thermoplastic resin soak and sure on the inner circumferential surface of the dowel hole by inserting the same into the insert hole.

CONSTITUTION: A nozzle 3 is inserted into a dowel hole 2 provided on the surface of a lightweight board 1 and molten thermoplastic resin 4 is injected into the dowel hole 2. A gap is provided between the dowel hole 2 and nozzle 3. A collar part 5 is provided on an appropriate part of the nozzle 3 to prevent extrusion of the resin outside of the cowel hole at the time of loading of the resin, the gap between the insert hole 2 and nozzle 3 is clogged and the nozzle 3 is compressed toward the dowel hole 2. The collar part 5 functions also as locationing device deciding an insertion depth of the nozzle 3. With this construction, the dowel hole is reinforced and connection strength of said hole is improved.