Three former executives of the operator of the crippled Fukushima Daiichi nuclear power plant have pleaded not guilty over the March 2011 accident.
Nuclear meltdowns occurred at the plant after it was hit by a giant earthquake and tsunami on March 11, 2011.
The defendants are former Tokyo Electric Power Company Chairman Tsunehisa Katsumata and former Vice Presidents Ichiro Takekuro and Sakae Muto.
They are accused of professional negligence resulting in the deaths of 44 people, including hospital patients forced to stay at evacuation shelters for a long period.
At the start of the trial at the Tokyo District Court on Friday, Katsumata apologized for the serious accident, and causing a great nuisance and concern.
But he said it was impossible to predict the tsunami, and the nuclear accident that followed, at the time.
Takekuro and Muto also offered apologies but pleaded not guilty.
Points of contention will likely include whether the defendants were able to predict that a huge tsunami would hit the plant, and whether the accident could have been prevented if proper steps had been in place.
This is the first trial concerning criminal responsibility for an accident at a nuclear power plant.
Public prosecutors decided not to file charges against the 3 former executives in 2013. But they were indicted in February last year by court-appointed lawyers in line with the decision by a prosecution inquest panel of randomly selected citizens.