Parts of northeastern Japan are still off-limits due to high radiation levels after the Fukushima nuclear plant was devastated by a huge earthquake and tsunami, triggering meltdowns and spewing radiation.
Anti-nuclear sentiment is high, with advocacy groups in Tokyo and other cities calling for radiation monitoring at schools and other public facilities.
"The threat from the nuclear accident cannot be seen by the human eye and continues to be a concern for many people, especially for mothers with small children," said Softbank founder and president, Masayoshi Son, standing in front of an aerial photo of the crippled plant.
The smartphone in the company's "Pantone" series will come in eight bright colors and include customized IC chips made by Sharp Corp that measure radiation levels in microsieverts per hour.
The phone, which goes on sale this summer, can also keep track of each location a user tests for radiation levels.
Son, who emerged as an outspoken critic of nuclear power and advocate of renewable energy sources after last year's quake, told vendors and reporters that the smartphone was more portable and user-friendly than conventional Geiger counters.
Softbank, Japan's No. 3 mobile phone operator, has not set a price on the new smartphone yet, but Son told reporters after the event that it would be in an affordable price range.
(Reporting by Mari Saito; Editing by Linda Sieg and Ron Popeski)