Workers at the crippled Fukoshima Dai-ichi nuclear complex stopped the leak of highly radioactive water into the ocean on Wednesday.
Radiation levels off the coast have steadily dropped. Testing has revealed levels now measure hundreds of times the legal limit rather than thousands.
"Right now, just because the leak has stopped, we are not relieved yet," Chief Cabinet Secretary Yukio Edano warned. "We are checking whether the leak has completely stopped, or whether there may be other leaks."
The facility's workers are still trying to cool the endangered reactors and bring them under control. That mission has been hampered by highly contaminated water that is pooling throughout the plant, making it difficult or impossible to access some areas.
The Tokyo Electric Power Company has been trying to come up with solutions to the pooled water, including using an empty warehouse to contain it.
"We must carefully check and repair the facility to make the water will not leak out and affect the environment," said Hidehiko Nishiyama, a spokesman for Japan's Nuclear and Industrial Safety Agency.
Engineers are preparing to inject nitrogen into the reactor to prevent any more hydrogen explosions. Nitrogen can prevent highly combustible hydrogen from exploding - as it did three times at the compound in the early days of the crisis following the 9.0 magnitude earthquake and resulting tsunami on March 11.
Nuclear officials said there was no immediate threat of more explosions.
"The nitrogen injection is being considered a precaution," Nishiyama said.