Japan has passed legislation that allows the defence chief to order the interception of incoming missiles without seeking Cabinet permission.
The bill was approved by the upper house in a 126-94 vote, and will now be made law. It was passed in the more powerful lower house last month.
Correspondents say there are concerns that the present system would waste time in the event of an attack.
Japan's pacifist constitution renounces using force in international disputes.
Japan's government has been reconsidering its defence policy, amid concern about the nuclear ambitions of its neighbour North Korea.
Japan is due to take part in six-nation nuclear talks with the Stalinist state next week in Beijing.
Pacifist policy
Under the new law, missiles can be launched without permission of the prime minister or the Cabinet only in an emergency.
If the situation is not urgent, defence personnel will continue to follow current procedures.
Supporters of the bill argued that the legislation was needed to respond quickly to an attack.
"If a missile comes flying into Japan, we have to shoot it down to protect the lives and property of the Japanese people before we can mobilise our defences," Defence Agency chief Yoshinori Ono told reporters.
But critics are concerned the legislation will give Japan's military too much power. They also fear the country's pacifist policy, in place since World War II, will be threatened.
Japan has been undergoing a review of its defence policy, and last year it eased restrictions on arms exports to allow greater collaboration with the US in areas of missile defence.
Tokyo says a review is necessary due to the world's changing security situation.
In 1998, Japan was shocked when North Korea fired a ballistic missile over the country.