To provide an internally inserted pipe stiffening method capable of securely preventing deformation such as dents and crushing which is caused at the time of injection and filling of a fixation filler to the surrounding of an internally inserted pipe because at this time, the internally inserted pipe is pressed and supported from its inner direction under pressure equal to injection pressure of the fixation filler.

An internally inserted pipe stiffening device 14 which uses, as its core member, a bag body 12 comprising a nontransparent material expandable by pressurization and filling of predetermined fluid is internally inserted into the internally inserted pipe 18 inserted into and supported by a sheath pipe 16 in the diameter contracted state at least over the overall length of the target portion of the sheath pipe 16. The core material (bag body 12) of the internally inserted pipe stiffening device 14 is pressurized and expanded under pressure equal to injection pressure of the fixation filler 21 injected and filled in an air gap 22 between the internally inserted pipe 18 and the sheath pipe 16. Due to this, the internally inserted pipe 18 is pressed and supported from its inner side by the pressure.