The factories are among those that were stopped immediately after the magnitude 9.0 earthquake struck, but were not heavily damaged.
Panasonic reopened its Koriyama factory in Fukushima prefecture.
It is the fourth Panasonic facility to reopen. On Tuesday, the company’s Utsunomiya consumer electronics factory and Yamagata factory partially restarted production.
NEC subsidiaries started work at four locations on Wednesday: NEC Tohoku in Iwate, NEC Wireless Networks in Fukushima, and NEC Tokin and NEC Infrontia Tohoku in Miyagi.
Both the Panasonic and NEC factories lie about 60 kilometers from the stricken Fukushima Daiichi nuclear power plant.
Japanese officials have imposed a 20 kilometre exclusion zone around the power plant and told people within a 20 kilometre to 30 kilometre radius to stay indoors. Those outside the area do not need to take any special precautions against radiation, according to the Japanese government. In contrast, some foreign embassies in Tokyo have advised citizens to stay at least 80 kilometres from the nuclear power station.
Factories across a wide region of Japan currently face problems securing materials and components. The usually smooth-running Japanese supply chain has been heavily disrupted by the earthquake and subsequent tsunami.
Rolling blackouts are also causing a headache for manufacturing companies. Tokyo Electric Power has introduced power rationing in regions around Tokyo to help cope with a big drop in generation capacity after several power plants were knocked offline by the quake.