The launch pad has been picked up on satellite images
Japan says it is deploying missile interceptors to destroy any parts of a North Korean rocket that might fall on its territory.
North Korea has said it will launch a satellite into orbit next month.
South Korea, Japan and the US say the launch is cover for a test of the Taepodong-2 ballistic missile.
The US said a launch would violate UN Security Council resolutions. Russia said North Korea should "abstain" from testing any missiles.
Japan's Defence Minister Yasukazu Hamada issued the orders to mobilise Japan's missile defence shield after a meeting with Prime Minister Taro Aso and cabinet ministers.
"We will do our best to handle any flying object from North Korea in order to assure the Japanese people's safety and security," said Mr Hamada.
"A satellite or a missile - we are displeased with anything that is going to fly over our land, and such an action must be stopped."
It is the first time that constitutionally pacifist Japan has deployed the shield. The country's military is also expected to deploy warships off its coast.
North Korea says it intends to test-fire the rocket between the 4 and 8 April.
The trajectory issued by Pyongyang shows the rocket will pass over Japan, with the first booster stage landing in the sea to the west, the second in the Pacific Ocean to the east.
The interception is only likely to be activated if the launch does not go as planned and debris appears to be falling towards Japan.
Russian Deputy Foreign Minister Alexei Borodavkin told journalists that the launch had led to increased tensions in the region, "and this is why it would be better if our partners in North Korea abstained from this".
Japan revised its Self-Defence Forces Law in 2005, legalising possible interceptions of ballistic missiles.
But the country's pacifist constitution does not allow it to intercept a missile if it is clearly heading elsewhere.
The Japanese government had previously warned it would try to shoot down any missile or debris that threatens to hit its territory.
North Korea has said it would regard any rocket intercept as an act of war.