Authorities earlier moved tens of thousands of people out of the area within 20 km (12 miles) of the stricken Fukushima No. 1 plant.
Workers have been battling a nuclear emergency there since last Friday`s huge earthquake and tsunami cut power to reactor cooling systems.
"I have been informed that the figures monitored today (outside the zone) were not anything that would harm human health immediately," spokesman Yukio Edano told a briefing.
At the plant, 250 kilometres (155 miles) northeast of Tokyo, radiation levels rose sharply earlier in the day -- prompting a brief evacuation of workers -- but fell soon afterwards.
Edano said the level was stable around 1.5 millisieverts near the front gate at 4 pm (0700 GMT).
He said workers were preparing another attempt to pump water around overheating reactor fuel rods to cool them down.
But media reports citing defence ministry officials said military
helicopters had postponed an attempt to drop water on the stricken plant because of the high radiation level.(*)