The case on Friday comes as Japan and other countries try to resolve a crisis over North Korea's nuclear weapons ambitions.
It also comes after the trade ministry tightened export controls to halt the export of technology that could be used to make weapons of mass destruction.
The ministry imposed the ban against ID Support Inc, a company in Niigata in northern Japan, which exported an inverter to North Korea via China last November.
Inverters are used in industrial washing machines, but can also be adapted for use for nuclear arms development, said Mitsuyoshi Shimoseko, a deputy director for the trade ministry's security export inspection office.
"There is a possibility that it could be used during a process to enrich uranium," Shimoseko said.
"I thought too easily that it was only a part for a washing machine, and this is what happened"
ID Support chief
The company had shipped the item without obtaining permission from the trade minister, the ministry said.
The transaction violated Japan's Foreign Exchange and Foreign Trade Law.
Shimoseko said the export of one inverter by itself would not pose major problems and that it wasn't clear whether the inverter, which was returned to Japan in December after spending less than a month in North Korea, was actually used for arms development.
The device, which measures about 10 cm by 15 cm, was taken to North Korea in hand luggage carried by an associate of ID Support chief Yoshifumi Yoshihara, Shimoseko said.
Yokohama district court, near Tokyo, sentenced the two to suspended prison terms in May. Yoshihara, who said his company sells food and household goods mostly in Niigata, said the inverter was intended for use in a washing machine in North Korea.
"I thought too easily that it was only a part for a washing machine, and this is what happened," Yoshihara said in a telephone interview.
In March, the ministry imposed a three-month export ban on Meishin, a Tokyo trading house. The firm had tried to ship equipment to North Korea via Thailand that could be used to develop nuclear arms or missiles.
The crisis over the North's nuclear ambitions began in October 2002, when US officials said North Korea had admitted it was working on a secret programme to enrich uranium for weapons. North Korea has denied the programme exists.