SEOUL:Shares in the management agency of K-pop sensation BTS more than doubled on their stock market debut Thursday, making an instant multi-billionaire of its chairman and boosting the seven band members' own fortunes.

The initial public offering of shares in Big Hit Entertainment saw staggering demand, with the public section oversubscribed more than 600 times and applicants receiving only a tiny fraction of their requests.

The firm's centrepiece asset BTS have risen to global stardom in recent years, cementing their prominence in the world's biggest music market in August with their all-English track "Dynamite" topping the US Billboard Hot 100.

The IPO price was set at 135,000 won ($118) but opened at double that on the Kospi exchange and within minutes hit its daily limit of 351,000 won, platforms showed.

It later slipped back but still had a market capitalisation of 10 trillion won $8.7 billion putting it among South Korea's top 40 most valuable companies, and ahead of cosmetics-maker Amore Pacific.

Big Hit founder and CEO Bang Si-hyuk who is retaining a stake of more than 36 percent of the firm was worth $3.8 billion at the peak, according to Bloomberg News.

At a listing ceremony, he thanked "all the fans who have always loved and believed in Big Hit's artists and content" and "our remarkable artists of whom we're so proud".

The flotation also boosted the BTS members' own wealth, Bang gave each of them more than 68,000 shares in August, worth around $20 million at the day's high and totalling 1.4 percent of the company.

Analysts had expected the shares to power upwards.

"Considering all the information about the firm now available, the IPO price could be the lowest price we will ever see," Park Sung-ho of Yuanta Securities told AFP.

Some investors cashed in straight away. "I received two shares and just sold them. With 260,000 won in profit, I will just buy a winter coat," said one poster on the South's biggest internet portal Naver.

Others cautioned that the phenomenal rise was unsustainable. "It is rising rapidly. And it will soon fall rapidly," said one.

- Reporting for duty -

There is one inescapable hurdle looming for the newly listed agency: mandatory military service for all seven boy band members, who made their debut in 2013.

South Korea requires all able-bodied men to serve in uniform to defend it against security threats from the nuclear-armed North, usually for 18 months.

Under the existing conscription laws, BTS member Jin, real name Kim Seok-jin aged 27, will have to report for duty by the end of 2021.

The other six members, born between 1993 and 1997, will have to follow in the coming years.

South Korea is currently debating exemptions for stars such as BTS, who have been at the forefront of the Korean Wave cultural phenomenon in recent years and are estimated to generate billions for the country's economy.

But no such dispensations have so far been granted to pop stars, and if they are forced to leave the stage it could blow a hole in Big Hit's finances: BTS were responsible for 97 percent of its revenues last year, according to the IPO prospectus.

Big Hit acknowledged their anticipated absence as a "risk factor".

It is looking to expand activities such as "contents licensing sales" that do not require the artists' "direct participation" to try to cushion the blow.

But it acknowledged their time off-camera "could have a negative impact on the company's profitability and growth".