WASHINGTON, April 7 - A military lawyer for a detainee at the Guantánamo Bay Naval Base in Cuba has filed the first lawsuit directly challenging the military tribunal system that has been set up to try prisoners including his client.

The lawyer, Lt. Cmdr. Charles Swift, filed the suit in his own name on Wednesday in Federal District Court in Seattle, his hometown. It asserts that the Bush administration's plans for his client violate the Constitution, federal law and the nation's obligations under the Geneva Conventions.

A Defense Department spokesman, Maj. Michael Shaver, declined to comment, saying Pentagon lawyers were still reviewing the papers.

The suit is another avenue of contesting the administration's plans to use military tribunals to try Guantánamo detainees, captured in the war in Afghanistan. The Supreme Court is scheduled to hear arguments on April 20 on a different challenge, dealing mainly with whether the prisoners at Guantánamo are beyond the reach of American law because the naval base is not United States sovereign territory.

Commander Swift represents Salim Ahmed Hamdan, one of six prisoners at Guantánamo who have been designated by President Bush as subject to charges before a military tribunal. Commander Swift said Mr. Hamdan, whom he met for the first time on Jan. 30, had authorized him to file the suit on his behalf because he himself had no access to the courts. Prof. Neal Katyal of the Georgetown University Law Center is in turn acting as Commander Swift's lawyer.

The suit asserts that Mr. Hamdan, a Yemeni, was never involved with Al Qaeda or with any military action against American forces in Afghanistan. He is described as a Muslim pilgrim who went to Afghanistan on the way to Tajikistan. Failing to get there, the papers say, he took a job as a driver on Osama bin Laden's Afgan farm and later became a driver for Mr. bin Laden himself.

The tribunal system set up by the administration does not provide for review in any civilian courts. Appeals may be taken only up the military chain of command.

The new suit, before Judge Robert S. Lasnik, asserts that the Constitution guarantees civilian court review of the military justice system. Military officials have contended that the prisoners are unlawful enemy combatants and as such are not entitled to the protections of American law or international treaties.