PROBLEM TO BE SOLVED: To efficiently assemble reinforcements by erecting vertical steel along a skeleton, mounting reinforcement assemblies from the outside to form reinforcing materials at site, and placing concrete in a form to construct a tower structure. SOLUTION: An inner precast slab 9 is erected on a bridge pier foundation, vertical steel 3 is placed on a steel projecting from the bridge pier foundation, they are fastened together by welding or by bolts or rivets through joint plates. Reinforcement assemblies 4 are mounted on the vertical steel 3 from the outside, and after the lower parts are connected to reinforcements projecting from the bridge pier foundation, an outer precast slab 10 is erected. Concrete is placed in a space between the inner and outer precast slabs 9, 10 to construct the skeleton 2 of the bridge pier 1. The reinforcement assembly 4 is formed by arranging helically surrounding reinforcements 6 around a plurality of vertical reinforcements 5. Belt reinforcements are previously buried in the inner/outer precast slabs 9, 10, but they can be fitted in the exposed state inside. Hereby the used reinforcement quantity can be remarkably reduced, and the assembling work can be saved in labor and made efficient.