A commission of experts mandated by the government has assessed "very critically the economic aspects" of Opel's request for more than €1 billion ($1.23 billion) in loan guarantees, Brüderle said.
The minister added however in comments made to German trade unions in Berlin that he had not yet seen the commission's conclusions, which will be submitted as recommendations.
But Brüderle added that "it is a secret for no one that I have been sceptical about the entire dossier from the beginning."
A second national commission comprised of government representatives is to meet Friday to decide whether to approve the guarantees for Opel.
The European unit of US auto giant General Motors has asked for €1.8 billion in loan guarantees from various governments to back a restructuring plan that would eliminate 8,300 jobs from a European total of around 50,000.
Germany, which would split its share of the aid among federal and state governments, has been very reticent to commit itself.
Brüderle said he must confer again with Chancellor Angela Merkel before taking the final decision, which is expected before next week.