PURPOSE: To make the cross section of a building frame small and to make economical bar arrangement possible by a method wherein a metal joint for reinforcing bars in different diameters made up by integrally joining a steel bar in large diameter and a steel bar in small diameter is used, and reinforcing bars in an RC column and an RC beam are joined therewith.

CONSTITUTION: A metal joint A for reinforcing bars in different diameters is made of cast steel and is constructed by connecting a steel bar 1 in large diameter integrally with a steel bar 2 in small diameter. The steel bar 1 in large diameter is welded (W) to a reinforcing bar 3 in large diameter in an RC column B and an RC beam C, and the steel bar 2 in small diameter is welded to a reinforcing bar 4 in small diameter in the RC column B and the RC beam C. Thereby, reinforcing bars in different diameters can be used optionally in a continuous building frame, and the diameter in the middle part of the reinforcing bar can be made small while the number of reinforcements in the RC column and the RC beam is kept unchanged.