A Japanese man released by North Korea after being detained for an unknown reason returned to Tokyo via Beijing on Tuesday following Pyongyang's decision to deport him.

Tomoyuki Sugimoto, who visited North Korea as a tourist, had been held for a criminal investigation until the country "decided to leniently condone him," the state-run Korean Central News Agency said in an English-language report on Sunday.

The nature of the alleged illegal activity was not specified. Pyongyang decided to release him "on the principle of humanitarianism," KCNA said.

According to Japanese government sources, Sugimoto is 39 years old.

The development comes as Prime Minister Shinzo Abe is looking to find a path to a summit with North Korean leader Kim Jong Un.

Foreign Minister Taro Kono said at a press conference on Tuesday the government is planning to question the man, who was taken into custody earlier this month but declined to elaborate further on the issue, including the North's intention.

A Kyodo News reporter spoke to Sugimoto on the plane to Beijing from Pyongyang on Tuesday morning, during which he confirmed his identity. The plane was operated by North Korea's national flag carrier Air Koryo.

At Pyongyang airport, Sugimoto boarded the plane and sat in an economy-class seat with other tourists and businesspeople. Clad in a black hat, dark glasses and black pants, he kept his eyes lowered most of the time aboard.

During the flight, Air Koryo staff kept a close watch on Sugimoto from behind. He had a cup of tea but no in-flight meal.

(Tomoyuki Sugimoto)

The plane had many Japanese aboard, including university students who had participated in exchange events with North Korean students in Pyongyang in August.

Once Sugimoto arrived at Beijing Capital International Airport, a throng of waiting reporters surrounded him.

Japan has been investigating why Sugimoto was detained, while continuing to communicate with North Korea through a diplomatic channel in Beijing, a source close to the matter said. Tokyo and Pyongyang have no diplomatic relations.

A Japanese government source said earlier this month that a man thought to be a videographer in his 30s from Shiga Prefecture in central Japan was detained in North Korea.

The man may have been suspected of videotaping a military facility when he visited the western port city of Nampo with a tour group, the source said.

The man was visiting North Korea on a tour arranged by a China-based travel agency, the source said, adding that there is unconfirmed information about him having visited the country in the past.

North Korea deported Sugimoto after a relatively short detention period -- three weeks -- while Tokyo urged Pyongyang to release him.

But many analysts are skeptical Sugimoto's release will pave the way for a summit between Abe and North Korean leader Kim, given that Pyongyang has stepped up criticism of Tokyo.

Although Abe has said tackling the issue of Japanese nationals abducted by North Korea in the 1970s and 1980s is his "life's work," Pyongyang has repeatedly said the issue has been "resolved," showing no intention of holding dialogue with Tokyo on the matter.

The Japanese government has requested that all its citizens refrain from traveling to North Korea, as part of sanctions against the country for developing and testing nuclear weapons and ballistic missiles in defiance of international warnings.

In 1999, a Japanese newspaper reporter was taken into custody in North Korea on spying charges and detained for about two years.

A Japanese trader was detained in 2003 on suspicion of smuggling drugs. It took around five years and three months before he was allowed to leave North Korea.