Speaking to the Reuters news agency, Shinji Kinjo revealed the leak is exceeding legal limits of radioactive discharge.
Countermeasures planned by Tokyo Electric Power Co (TEPCO), the operator of the Fukushima nuclear complex, are only a temporary solution, Mr Kinjo added.
"Right now, we have an emergency," he said.
TEPCO has been struggling to contain hundreds of tonnes of groundwater entering the plant everyday - water that quickly becomes contaminated.
But the company has also been roundly condemned for failing to make public leaks of radioactive water into the Pacific, despite knowing about it.
In the early weeks following the 2011 earthquake and tsunami, the Japanese government allowed TEPCO to dump tens of thousands of tonnes of toxic water into the Pacific in an emergency move.
How do nuclear reactors work?Nuclear power stations are a major source of energy for many countries around the world.
That prompted heavy criticism by neighbouring countries as well as local fishermen, and the company has since promised it would not dump irradiated water without the consent of local townships.
In a bid to prevent more leaks into the bay of the Pacific Ocean, plant workers created an underground barrier by injecting chemicals to harden the ground along the shoreline of one of the reactor buildings.
But that barrier is only effective in solidifying the ground at least 1.8 metres below the surface.
By breaching the barrier, the water can seep through the shallow areas of earth into the nearby sea.
More seriously, it is rising toward the surface - a break of which would accelerate the outflow.
The regulatory task force overseeing accident measures of the Fukushima station, which met last Friday, "concluded that new measures are needed to stop the water from flowing into the sea that way," Mr Kinjo said.
abc.net.au 6 Aug 2013
The Japanese government and the associated corporations should be charged with genocide by the world courts.
The Japanese has a figure of safe radiation level exposure prior to the disaster.
After the disaster occurred, and the real level of radiation was made public the government went into panic control and changed the original figure in order NOT to create a mass panic, in effect secretly condemning the people to death.
This fact has been auspiciously omitted by the corporate media, and the world courts.