PURPOSE: To enable quick detection of a film breakage at a joint of a clad steel plate on-line in a non-destructive manner by leaving the sheet wound with a coil after laminated with an insert paper or the like absorbing water so as to promote the generation of rust at the film breakage portion.

CONSTITUTION: An insert paper or cloth A coiled with a payoff reel 1 is fed to a tension reel 14 through deflector roll 16 absorbing water B sprayed through a spray nozzle 7 by way of a water feed branch piping 6 from a water tank 2 to be laminated on a clad steel sheet C and then, the work is wound up. The wound coil is left alone for about 30W50hr to promote the generation of rust at the film breakage portion and then, unwound to check for risen on the surface. This enables the detection of any fine film breakage along with a higher inspection efficiency.