PURPOSE: To enhance vertical strength of a newly constructed or existing concrete pillar by using PC steel materials in multiple stages horizontally and tensing them and by giving horizontal compressive stress to inside concrete as prestress.

CONSTITUTION: In case of a newly constructed pillar, horizontal compressive stress is generated by tightening neighbouring concrete in the centripetal direction by using PC steel materials 5 formed with, for example, ring one in multiple stages and tensing them in intervals of horizontal reinforcing bars of the pillar. In this case, end parts of the PC steel materials 5 are exposed from the concrete, tensed by a hydraulic jack, or the like after the deposited concrete is hardened and fixed on the concrete surface. Consequently, when a vertical load is added on the pillar 1 and vertical compressive stress is generated, it becomes triaxial stress, and vertical strength is strengthened larger than in the case of only conventional vertical stress. Even in case of strengthening an existing concrete column, the PC steel materials 5 treated with rust preventives are wound around its outer periphery and tensed, and compressive stress in the centripetal direction is given to the inside concrete.