A main reinforcing bar has a strength transition portion between a normal strength portion and a high strength portion. The high strength portion is arranged in a joint section. The boundary between the normal strength portion and the strength transition portion is configured as a deigned point. The designed point is designed such that, at the time of an earthquake, the main reinforcing bar yields at the designed point before the main reinforcing bar yields at the root of the beam at of the joint section. The boundary between the high strength portion and the strength transition portion is located in the joint section, and the root of the beam is located at the strength transition portion. The strength of the strength transition portion at the root of the beam is equal to or higher than the required strength.