PURPOSE: To perform building of a smooth concrete roof at a low cost by erecting a rafter on support members arranged on a reference surface, placing concrete on a basis of the rafter, and mounting roof plates on the rafter.

CONSTITUTION: A plurality of support members 12... are arranged at intervals of a proper distance on a reference surface produced by a temporary forms plate 11, and a rafter 13 is erected on the support members 12, arranged in a given direction, through respective fixing tools. Concrete is placed on a reference surface on a basis of the rafter 13 to build a concrete body 14, and roof plates 17... are mounted on the rafter 13. The rafter 13 is provided with upper surface parts 13a and 13a to provide a first level during placing of concrete and flange parts 13c and 13c to provide a second level. This method facilitates formation of an uniform concrete surface having no uneven surface.