This paper presents an algorithm of adaptation for a case-based reasoning system with cases and domain knowledge represented in the description logic ALC. The principle is to first pretend that the source case to be adapted solves the current target case. This may raise some contradictions with the specification of the target case and with the domain knowledge. The adaptation consists then in repairing these contradictions. This adaptation algorithm is based on an extension of the tableau method used for deductive inferences in ALC.