To improve work exectiveness by embedding a flexible connecting material in an annular shape in a mutually butting end surface of a box culvert, exposing its part from the end surface, and arranging an electric heating body in an exposed part.

A flexible connecting material 4 composed of thermoplastic synthetic resin or the like is embedded in an annular shape in an end surface of a box culvert 1 by exposing its part. Next, for example, the connecting material 4 is formed of a cylinder part 5 exposed from the end surface, plate- like parts 6 and 7 to be embedded and a cylinder part 8, and a heating wire 10 is embedded in the cylinder part 5, or an electric resistor is adhered to the outside of the cylinder part 5. Next, at execution work time, after the opposed cylinder part 5 of the continuously arranged culvert 1 is contacted and is made flat, it is heated by carrying an electric current to the heating wire 10, and after the cylinder part 5 is melted, current-carrying is stopped, and a melting part is hardened, and is integrally joined. Therefore, it can be easily and quickly welded without using a hot iron, and since it melts uniformly, water stop performance can be improved.