Large amounts of solidified lava-like rocks and lumps in layers were seen underneath its unit three reactor.
If confirmed, it would be a major milestone in the clear-up operation.
The power plant was hit by a tsunami in 2011, causing the most serious nuclear accident since Chernobyl.
More than 200,000 people were forced to leave their homes because of nuclear contamination concerns when three reactors failed at the plant after it was flooded.
Parts of the damaged reactors are still highly contaminated, with radiation and robotics playing a crucial part in the clean-up.