Flights in and out of Sendai Airport were cancelled after the 250kg (550lb) device was uncovered during construction of a runway.
The bomb was identified as American-made and is believed to be a dud from World War II.
However, it appeared to have a working detonator, and a military bomb squad is considering whether to move the bomb or explode it on the spot.
Sandbags were piled up around the bomb, and local officials were discussing possible evacuations of nearby homes.
They added that the area had been sealed off and there was no immediate concern that the bomb would explode.
Japanese cities were bombed heavily during World War II, so finding dud bombs is not unusual, but their rusty condition can make them prone to detonate when moved.
Sendai Airport is currently being reconstructed after receiving damage from last year's tsunami.
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