* Chung Eui-sun buys $259 mln worth of Hyundai Motor shares
* Firm says stake purchase is to protect shareholder interests (Updates with details of stake purchase, background)
SEOUL, Nov 12 (Reuters) - South Korean car maker Hyundai Motor Co's Vice Chairman and heir apparent Chung Eui-sun has bought 300 billion won ($259 million) worth of the firm's shares and lifted his stake, as he prepares to eventually assume control of the Hyundai Motor Group.
Hyundai Motor said in a regulatory filing on Thursday Chung bought 1.85 million shares at 162,500 won per share from unlisted Hyundai Samho Heavy Industries Co Ltd, raising his stake in the car maker to 2.28 percent. Hyundai Motor Co is the main affiliate in Hyundai Motor Group, the country's No. 2 family-run conglomerate.
Chung had in September bought 500 billion won in Hyundai Motor shares from Hyundai Heavy Industries Co Ltd.
Investors have said the 45-year-old Chung, only son of current Hyundai Motor Group Chairman Chung Mong-koo, needs to hold more shares in key group affiliates in order to ensure a smooth succession. The younger Chung only held 6,445 shares in Hyundai Motor prior to the September stake purchase.
A Hyundai Motor spokesman said Chung bought the shares to protect shareholder interests and to ensure stable management of the company. Regulatory filings did not disclose how Chung financed the purchase but he recently sold shares in other Hyundai Motor Group affiliates.
Hyundai Samho, a shipbuilder, said in a regulatory filing dated Tuesday it is selling the Hyundai Motor shares to improve its financial health but had not disclosed the buyer. ($1 = 1,157.5800 won) (Reporting by Se Young Lee; Editing by Clarence Fernandez and Muralikumar Anantharaman)