To provide a segment assembling method for efficiently performing simultaneous construction of segment assembly using the same trapezoidal segments abreast with the advance of a shield machine and moreover shortening a boring and assembling cycle and allowing stable attitude control during simultaneous construction.

A segment ring is formed by a primary assembling process for assembling a plurality of trapezoidal segments 5a set in a forward direction with the short-side side facing forward in boring, alternately in a circumferential direction abreast with the boring while advancing the shield machine and adjusting the pressure of jacks other than the assembling range, and a secondary assembling process for inserting a plurality of trapezoidal segments 5b set in a reverse direction with the long-side side facing forward in boring, axially between the alternate trapezoidal segments 5a in the forward direction in the stopped state of the shield machine to assemble the trapezoidal segments 5b with the trapezoidal segments 5a in the forward direction.