Tokyo Electric Power Company says electricity will be restored to Chiba City and other areas in the northern part of Chiba Prefecture on Thursday.
The prefecture suffered widespread power outages after Typhoon Faxai made landfall near Tokyo early Monday.
The utility said on Thursday morning that the cities of Chiba, Yachiyo, Yotsukaido and Inzai will have power restored by the end of the day. Parts of Ichihara City will also have lights back on.
But other areas, mainly in the south of the prefecture, will have to wait until Friday or later.
TEPCO says 11,000 workers, including those from other power companies, are trying to restore electricity.
The utility attributes the delay in restoration work to the large number of fallen trees that are hampering efforts to fix facilities and check for damage. It says large vehicles are needed for some repairs, and fallen trees must be cut in some areas. Power lines have also incurred significant damage at multiple locations.
Some 340,000 households in Chiba Prefecture were without power as of 8:00 a.m. Thursday.
TEPCO says the massive blackout is the longest since the March 2011 earthquake and tsunami.