Japan's defence minister could soon be given the power to shoot down incoming missiles without first seeking permission from the cabinet.
Japan is buying a sophisticated anti-missile system from the US
The draft bill was approved by the cabinet and is expected to go before parliament later this week.
Correspondents say there are concerns that the present system would waste time in the event of an attack.
A major defence review is under way in Japan, amid concern about the nuclear ambitions of its neighbour North Korea.
Chief Cabinet Secretary Hiroyuki Hosoda said the defence chief could only give the order to launch missiles if the country was under attack.
Currently, permission is needed from parliament.
"Lately, there are more countries equipped with missiles. We need to deal with the situation immediately if a missile were to be launched at Japan," Mr Hosoda told a news conference.
Under the bill, missiles could be launched without permission only in an emergency and the prime minister must notify parliament.
If the situation is not urgent, the defence minister would follow current procedures.
Japan has been undergoing a review of its defence policy and in December 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.
Last week, Pyongyang publicly said for the first time that it had nuclear weapons and was pulling out of multi-party talks on the nuclear issue.
Although Japan has restated its constitutional pledge never to threaten other nations, critics fear the country's pacifist policy, in place since World War II, is being threatened.