* First gov't share sale since the company's 2015 IPO
* Japan Post will buy back Y100 bln worth own shares (Adds market comments, background)
TOKYO, Sept 11 (Reuters) - The Japanese government will sell an additional stake in Japan Post Holdings Co Ltd worth around $12 billion, a regulatory filing showed on Monday, the first sale since the company's mammoth 2015 listing.
The government will sell about 914 million shares in Japan Post Holdings, according to the filing with the Kanto Local Finance Bureau.
The offering would be worth around 1.3079 trillion yen ($12.1 billion) based on Monday's closing share price of 1,321 yen and including options to sell additional shares in the event of strong demand.
The price will be determined between Sept. 25 and Sept. 27 and the delivery of the shares will take place four business days later, the filing showed.
Japan Post also said it will buy back its own shares worth up to 100 billion yen.
Japan Post and its two units Japan Post Bank Co Ltd and Japan Post Insurance Co Ltd made an unprecedented three-way initial public offering in November 2015, from which the government raised about $12 billion.
The government has said it eventually plans to raise $36 billion through the sale of shares in Japan Post and its two financial units, partly to fund reconstruction of areas in northern Japan hit by a catastrophic 2011 earthquake and tsunami. ($1 = 108.5100 yen) (Reporting by Taiga Uranaka and Chris Gallagher; Editing by Christopher Cushing)