To provide an arrangement method of a column main reinforcement in an earthquake strengthening construction method of an existing RC column and an earthquake strengthening structure of the existing RC column, capable of easily and efficiently earthquake-strengthening the existing RC column.

A plurality of through-holes 5 are opened at positions near the existing RC column 1 of respective story floor slabs 4. At the positions near the existing RC column 1 of respective stories, a plurality of main reinforcements 2 of the respective stories are made to pass through the through-holes 5 of the respective stories and arranged. The main reinforcements 2 of the upper and lower stories are joined with each other to form the plurality of column main reinforcements which are continued in one line. The main reinforcements 2 of the upper and lower stories are joined with each other on the same axis through a joint metal fitting 6. The plurality of column main reinforcements 2A are deformed roughly in an arched shape respectively and fixed at positions marked beforehand on the respective story floor slabs 4. The column main reinforcements 2A are fixed by a plurality of positioning metal fittings 7 to the floor slabs 4 of the respective stories. Around the plurality of column main reinforcements 2A, high strength concrete or high strength shrinkage-compensating grout mortar 11 is placed.