PURPOSE: To install a parting plate, which is used when the preceding element of an underground continuous wall is formed, correctly at the specified opening end of a main bored hole for the preceding element.

CONSTITUTION: A guide wall 3 is mounted onto the ground surface of a trench 1 formed through initial excavation so as to hold the trench 1. Guide grooves 4 extended in the depth direction at intervals corresponding to the unit length of a preceding element 14 are formed to the guide wall 3, lower soil and sand between the adjacent guide grooves 4 are mainly excavated and a bored hole 6 for the preceding element is shaped, and a pair of parting plates 7, lower end sections of which reach both sides of the opening end of the main bored hole 6 through the guide grooves 4 of the guide wall 3 are suspended and installed. Inwards expanded box-shaped sections 10 are formed previously to intermediate sections in each of the parting plates 7, and concrete is placed into the main bored hole 6 up to positions reaching the box-shaped sections 10 of parting plates 7. A narrow section at the upper end section of the preceding element 14 formed by the box-shaped sections 10 functions as a guide at a time when a bored hole for a subsequent element is excavated by a drum type cutter excavator 5.