Sarbanes-Oxley, the American law designed to stamp out corporate fraud, is prompting more companies to keep secrets in the bond market.

Siemens AG, the Australian retailer Woolworths Ltd., Miller Brewing Co. of Milwaukee, and at least 100 other companies are selling bonds that aren’t registered with the Securities and Exchange Commission instead of debt that requires more disclosure. The securities increased 50% in the past two years, five times faster than the rest of the American market, according to data compiled by Lehman Brothers Holdings Inc.

“It’s a darker world of the bond market,” a manager of $97 billion in fixed income, including unregistered bonds, at Loomis Sayles & Co. in Boston, Matthew Eagan, said. “It’s off the radar.”

The private bond sales are flourishing because companies face almost no penalty for keeping their finances away from the public. The millions of dollars in costs to comply with the Sarbanes-Oxley Act of 2002 can wipe out savings from public debt because investors demand only 11 basis points more in yield to buy unregistered securities, Lehman data show.

Sarbanes-Oxley requires companies to hire external auditors to evaluate their financial reports and the law applies to borrowers outside the country that want to sell securities to the American public.

The rule is consistent with efforts to make the bond market more transparent by requiring traders to report sales to an NASD computer system that distributes prices on the Internet.

Unregistered bonds make the market more opaque because trades aren’t reported on the NASD’s Trade Reporting and Compliance Engine. They can only trade between institutions.