To easily regulate the installation position and perpendicularity of a gatepost by erecting a cylindrical part in a hole provided in the vicinity of each end part of a plate longer than the distance between two gateposts, and inserting and supporting the gateposts in the perpendicular direction.

In a support piece 9 of a gatepost, a cylindrical part 2 is erected in the vicinity of each end part of a plate 8 longer than the distance between two gateposts, and a gatepost 1 is inserted in the cylindrical part 2 and supported in a perpendicular manner. In the cylindrical part 2, a hollow cylindrical member 7 is fitted in a hole selected among a plurality of holes 8a, and erected therein. As the execution procedure, pebbles are placed in a base hole at the position where the gatepost is erected, and the concrete is poured. The plate 8 is arranged on the base hole, and the cylindrical member 7 is erected. A lower end part of the gatepost is inserted in the cylindrical member 7 and inserted in the concrete. The level and the perpendicularity of the plate 8 are regulated, and the projection of the lower end part of the gatepost from the cylindrical member 7 is regulated and the concrete is cured. Then, the support piece 9 is lifted and detached. The position and the perpendicularity of the gatepost can be easily and accurately regulated by regulating the plate 8.