Softbank Corp., Japan's third- biggest mobile phone company, and Mitsui & Co. plan to build and start operating what may be the nation's biggest solar power plant.

Spurred on by Japan's incentive program for clean energy and Softbank Chairman Masayoshi Son's ambition to install more renewable energy in a country that last year experienced the world's worst nuclear disaster in a quarter of a century, the two companies today signed a pact to build 39.5 megawatts of capacity at a site in Tottori prefecture.

Tottori will be Japan's largest solar plant when it starts in July, Son said at a briefing in Tokyo today. The phone company also plans a 111-megawatt solar generator on the northern island of Hokkaido, which is scheduled to begin operations in fiscal 2014, according to Softbank.

Since the Fukushima nuclear disaster in March 2011, Japan's government has moved to ease the nation's reliance on atomic energy in favor of renewables and natural gas. According to a state incentive plan introduced this year, power utilities are obliged to buy electricity from renewable sources such as solar and wind at above-market rates. Softbank plans to invest in about 230 megawatts in renewables, mostly in solar.