PURPOSE: To prevent the surface layer of lining from separating or the like by a method wherein the quantity of lining material to be sprayed is controlled in accordance with the measured value of the thickness of lining so as to be able to ensure the total thickness necessary for obtaining the heat insulating and corrosion-resistant performances.

CONSTITUTION: A measuring gage 21 consists of a base plate 22a, which abutts against the inner peripheral surface of a smoke stack 1, a base plate 22, which serves as a base to abut against a plurality of transverse reinforcing bars 10a of a wire-netting 10, a stepping rule 23, which is fitted to the base plate 22 so as to extend itself in the longitudinal direction of the wire-netting 10 and parallel to the surface of the wire-netting 10 and to keep required distances la and (l) respectively from the inner peripheral surface of the smoke stack 1 and from the surface of the wire-netting 10. The fitting of both the base plate 22 and rule 23 is done by binding with each other by means of mechanisms 24 composed of an elongated hole and a bolt and nut so as to be able to adjust the required distances la and (l). The covering depth of lining and the total thickness of lining measured from the inner peripheral surface of the smoke stack 1, both of which are formed by spraying lining material over the wire-netting by rule of thumb, are corrected base upon the result of the judgement of the excess and shortage of the total thickness of lining and covering depth of lining by the edge of longitudinal end of the stepping pointer 23 as the criteria realized by abutting the measuring gage against the surface of the lower, adjacent and unlined wire-netting by means of a handle 25.