To provide a construction method of an underground structure capable of reducing a reconstructing joining member, by reducing a skeleton removing range and a ground improving range, when joining a tunnel and the other structure.

A ramp tunnel 13 having a width expanding segment 19 is arranged on the side of an existing main track tunnel 3 in the natural ground 1. A circular shape holding member 20 is arranged on the inner peripheral surface 16 side of the width expanding segment 19. An outbreak part 23 filled with a filler 25 is arranged on the outer peripheral surface 18 side of the width expanding segment 19. Next, while recovering the filler 25 in the ramp tunnel 13, the width expanding segment 19 is pushed out in the outbreak part 23. The improved ground is arranged around a joining expected part 8, and a form segment and the filler of the joining part are removed. Joining concrete is arranged in the joining part, and the filler, a segment 5a, a segment 14a and the circular shape holding member 20 of the width expanding segment 19 are removed.