Ryohei Shiomi said that officials were checking whether a meltdown had taken place at the Fukushima Daiichi power plant's Unit 1, which had lost cooling ability in the aftermath of Friday's powerful earthquake.
Shiomi said that even if there was a meltdown, it wouldn't affect humans beyond a six-mile (10-kilometer) radius.
Most of the 51,000 residents living within that radius have been evacuated, he said.
Earlier Saturday, Japan declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability.
Operators at the Fukushima Daiichi plant's Unit 1 scrambled to tamp down heat and pressure inside the reactor after the 8.9-magnitude quake and the tsunami that followed cut off electricity to the site and disabled emergency generators, knocking out the main cooling system.
Some 3,000 people within two miles (three kilometers) of the plant were urged to leave their homes, but the evacuation zone was more than tripled to 6 miles (10 kilometers) after authorities detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.
The government declared a state of emergency at the Daiichi unit -- the first at a nuclear plant in Japan's history. But hours later, the Tokyo Electric Power Co., which operates the six-reactor Daiichi site in northeastern Japan, announced that it had lost cooling ability at a second reactor there and three units at its nearby Fukushima Daini site.
The government quickly declared states of emergency for those units, too.
Japan's nuclear safety agency said the situation was most dire at Fukushima Daiichi's Unit 1, where pressure had risen to twice what is consider the normal level. The International Atomic Energy Agency said in a statement that diesel generators that normally would have kept cooling systems running at Fukushima Daiichi had been disabled by tsunami flooding.