The central issue in the case, which is an appeal from a decision of the United States Court of Appeals for the 11th Circuit in Atlanta, is whether Congress overstepped its constitutional authority by requiring almost everyone to obtain health insurance or pay a penalty. The Obama administration contends that this part of the law, often called the individual mandate, can be justified by the constitutional power of Congress “to regulate commerce” and “to lay and collect taxes.”

Beyond his apparent defense of Justices Thomas and Kagan, the chief justice used the report to fend off critics who say Supreme Court justices should be bound by the same code of judicial ethics that applies to other federal judges. He pointed out that the justices voluntarily follow the same rules, but said they should not be bound by them. Congress has also enacted laws concerning gifts and financial disclosure that are also applicable to Supreme Court justices.

The constitutionality of those laws remains an open question, Chief Justice Roberts wrote, adding that the justices nonetheless abided by them, including the “sometimes complex reporting requirements” for financial disclosure.

There have been missteps. Justice Thomas for several years failed to note the sources of his wife’s income on financial disclosure forms. After the watchdog group Common Cause raised questions, he issued amended forms last January. A court spokeswoman said at the time that the information had been “inadvertently omitted due to a misunderstanding of the filing instructions.”

Chief Justice Roberts said the justices also comply with the recusal law, though he added that “the unique circumstances of the Supreme Court” must be taken into account.

“There is only one major difference in the recusal process: There is no higher court to review a justice’s decision not to recuse in a particular case,” he wrote. “This is a consequence of the Constitution’s command that there be only ‘one Supreme Court.’ ”

That also means, he added, that recusal at the Supreme Court is particularly problematic.

“If an appeals court or district court judge withdraws from a case, there is another federal judge who can serve in that recused judge’s place,” he wrote. “But the Supreme Court consists of nine members who always sit together, and if a justice withdraws from a case, the court must sit without its full membership.”

“A justice accordingly cannot withdraw from a case as a matter of convenience or simply to avoid controversy,” he added. “Rather, each justice has an obligation to the Court to be sure of the need to recuse before deciding to withdraw from a case.”