To eliminate manual work under very bad environment and provide a chimney repairing device, high in working efficiency, by a method wherein repairing works in a chimney are unmanned.

In a chimney repairing device, a gondola 2 is hung elevatably in a chimney 1 by wires 3 through the upper smoke discharging port of the chimney 1 and tires 2i, pushed against the inner surface of the chimney, are provided at a plurality of places of the outer periphery of the gondola 2 so as to be capable of being advanced and retreated while rotary bodies 2d, 2t, which are rotated about the center of the inner surface of the chimney, are provided at upper and lower two places of the gondola 2. On the other hand, a mounting rod 2m is attached to one of the rotary body 2d by supporting the base end thereof through a bearing so as to be elevatable in the radial direction of the chimney while a rotary cutting device 2q is attached to the tip end of the mounting rod 2m and the tip end unit of the mounting rod 2m is connected to the other rotary body 2t through an expanding and contracting cylinder 2u to advance and retreat the rotary cutting device 2q freely in the radial direction of the chimney.