TOKYO - Japan's government is expected to nominate Masatsugu Asakawa, the country's former top currency diplomat, as its candidate to become next head of the Asian Development Bank (ADB), two people familiar with the matter said.

The top ADB post has always been held by Japan, which along with the United States is the bank's biggest shareholder, making Asakawa a strong candidate to take up the post though there is a chance China could put up a candidate.

The nomination will be put to vote by the members of the Manila-based multilateral lender. If selected, Asakawa could assume the post early next year, the sources said on condition of anonymity due to the sensitivy of the matter.

Asakawa will succeed incumbent Takehiko Nakao, a former top Japanese currency diplomat who has served since 2013.

Japan's Nikkei newspaper reported on Tuesday that Nakao will announce his resignation soon and that Japan's government will nominate Asakawa as his successor.

Japan has played a key role in creating the ADB to offer loans to developing Asian economies. Senior Japanese Ministry of Finance officials has thus dominated the post of the ADB president since its creation in 1966. Bank of Japan Governor Haruhiko Kuroda served as ADB president before Nakao.

The ADB faces increasing competition from the Asian Infrastructure Investment Bank (AIIB), a China-sponsored development bank, though the two lenders have said they are complementary partners in a region where demand for infrastructure funding remains huge.

Japan's finance ministry officials were not immediately available to comment.

(Reporting by Leika Kihara, Yoshifumi Takemoto and Takahiko Wada; Editing by Sam Holmes) ((leika.kihara@thomsonreuters.com; +813-6441-1828; Reuters Messaging: leika.kihara.reuters.com@reuters.net))