PURPOSE: To eliminate any need for cast-in place concrete and manufacture a spiral staircase with enhanced construction efficiency and accuracy based on a PC method.

CONSTITUTION: Specified dimensions for concrete for each step of a spiral staircase are secured. Form blocks 6 are assembled and loaded on a block mount at the position where the rear surface of the spiral staircase is formed in the shape of steps, thereby forming a form for the rear surface in the shape of steps. Reinforcements 12 and 13 are laid out while a miter form on the side surface of the spiral staircase 14 and a miter form 15 for the riser are mounted where concrete is placed.