"As long as we continue our sensible policies we needn't fear anything," Schäuble said at the government's open day in Berlin.
The minister said the work of the agencies were superior to government bodies that perform a similar task, but warned that it was not good that there were only three such companies in the western world.
Schäuble added that the agencies had made several serious errors, mentioning their failure to predict the 2008 financial crisis as an example.
The ratings agency Standard & Poor's recently deprived the US of its top AAA rating, though its main rivals Moody's and Fitch have decided not to.
Schäuble also took the opportunity to reiterate the government's refusal to countenance the idea of eurobonds. He said as long as the European financial policy was not unified and there was no European Union finance minister there could be no unified interest rates on European bonds.
He said varying interest rates were a motivation for governments to balance their books.