To provide a fitting structure of a baseboard for a stair, which exhibits a good appearance.

As for a fitting structure of a baseboard for a stair, end parts of a horizontal baseboard 4 and a vertical baseboard 5 corresponding to each other are diagonally cut in the length directions of the baseboards, and respective cut-ends are fastened and joined to each other. A coupler fitting groove 43 opened at the cut-end 41 is formed in the back side of the horizontal baseboard 4 and a coupler fitting groove 53 opened at the cut-end 51 is formed in the back side of the vertical baseboard 5. One side part 61 of a coupler 6 is fixedly fitted in the coupler fitting groove 43 of the horizontal baseboard side, while the other side part 61 of the coupler 6 is fixedly fitted in the coupler fitting groove 53 of the vertical baseboard side.