A device internally insulating a welded pipeline joint between pipes having an internal protective coating, in a first variant, includes a power actuator having a cylindrical elastic working member for radial expansion when excess pressure occurs inside its cavity. A cylindrical casing of elastic anti-adhesive material arranged coaxially on the member exterior can have channels for exhausting air and supplying a compound. In a second variant, the device sleevelessly insulates an annular space. A casing made of an anti-adhesive material is centrally reinforced by an elastic cord. Using a cylindrical elastic anti-adhesive casing coaxially arranged on the exterior of a power actuator elastic working member forms an annular space in the weld zone either using a protective sleeve, or by a reinforced casing portion. A vacuum in the space is filled by a compound. The elastic anti-adhesive material allows easy removal of the casing from the hardened compound surface.