By Agence France-Presse
Local police cleared the suspicious object and there were no flight cancellations, as one runway operated as normal during the three-hour closure, officials added.
"Contractors involved in drilling work at night found something like an explosive" in the soil, said a spokesman for Narita, Japan's main international hub east of Tokyo.
The builders described the object as "something like an unexploded shell".
Police said the risks of an explosion were low but they are investigating.
Narita handles 40 million passengers and 250,000 flights a year.