Japanese media said researchers found radiation levels of 3.35 microsieverts per hour along a street in the Setagaya district, west of the capital, 220 kilometres from Fukushima, much higher than previously reported levels.
According to calculations based on the Japanese science ministry's criteria, the equivalent annual dose in the hotspot would be 17.6 millisieverts, just below the 20 millisieverts per year threshold at which evacuation is obligatory.
The reading is also higher than levels measured recently at Litate, an area in Fukushima prefecture which has been evacuated.
Radiation levels have not fallen since efforts were made to decontaminate the area on 6 October and authorities are instructing children to avoid the walkway as they go to school.
The March 11 earthquake triggered a tsunami that tore into Japan's northeast coast, leaving 20,000 people dead or missing, while sparking meltdowns and explosions at the Fukushima Daiichi nuclear power plant.
The subsequent release of radiation forced the evacuation of tens of thousands from a 20 kilometre radius from the plant.