TOKYO, Sept 29 (Reuters) - Japanese shares fell on Tuesday for the first time in three sessions as telecom major NTT's bid to take complete ownership of its mobile unit, NTT Docomo, weighed on rival mobile network operators.

The Nikkei 225 Index fell 0.55% to 23,382.89 by 01:44 GMT. The broader Topix fell 0.83% to 1,648.07.

NTT said it was looking at taking full control of its wireless carrier business, NTT Docomo, in a tender offer that could be worth around 4 trillion yen ($38 billion).

The buyout will be discussed at a board meeting on Tuesday, NTT said in a statement after the Nikkei newspaper reported the deal. The value of the 34% of NTT Docomo's shares not owned by NTT is based on a 30% premium to Monday's closing price.

NTT Docomo's shares were untraded with a glut of buy orders, while NTT's shares fell 5.18%.

Rival mobile carriers also fell as the acquisition is expected to pile on pressure to lower mobile phone fees and increase competition for the roll out of 5G services. Shares of KDDI Corp lost 4.21%, while shares of SoftBank Corp fell 5.19%.

Japan's new prime minister, Yoshihide Suga, has made lowering mobile costs a policy priority, and the government is NTT's biggest shareholder.

Japanese stocks also fell as many shares went ex-dividend before the next round of corporate earnings.

The stocks that gained the most among the top 30 core Topix names were venture fund SoftBank Group Corp, up 2.3%, followed by Keyence Corp, up 2.25%.

There were 38 advancers on the Nikkei index against 182 decliners.

The volume of shares traded on the Tokyo Stock Exchange's main board was 0.41 billion, compared to the average of 1.14 billion in the past 30 days. (Reporting by Stanley White; Editing by Vinay Dwivedi)