PURPOSE: To simplify the attaching construction of a large thin-plate like building material such as a stone material.

CONSTITUTION: A wedge 18, gradually changed in its longitudinal thickness and provided with a single or plural laterally extended cutout grooves 24 formed at specified spaces apart along the longitudinal direction, is inserted between thin plate like building material 10 and a fitting face 12 from the thin wall part side to perform the positioning of the building material 10 together with a presser bar 20, and a part 28 protruding from the outer periphery of the building material 10 is broken at the cutout groove 24 and removed. After the building material 10 being temporarily fixed, the attachment of the following building material 10 is performed in the same method.