Outgoing Japanese Prime Minister Shinzo Abe has been admitted to hospital, a day after announcing his resignation.
Abe has been in the job for less than a year
Doctors said Mr Abe was being treated for a stomach complaint probably caused by extreme exhaustion and stress.
Japan's governing party has announced it will choose a successor to Mr Abe on 23 September.
Finance Minister Fukushiro Nukaga has said he will stand, but the most likely candidate, former Foreign Minister Taro Aso, has yet to make an announcement.
The new Liberal Democratic Party (LDP) leader is guaranteed to become prime minister, because the party controls parliament's lower house.
Mr Abe had been facing growing calls to quit since his party lost upper house elections in July, and opinion polls showed he was increasingly unpopular.
But there has also been speculation that health problems were partly behind his decision to step down.
"I hear that the doctor diagnosed him as suffering from extreme fatigue," Chief Cabinet Secretary Kaoru Yosano told reporters after Mr Abe was admitted to hospital.
It is unclear whether he will be able to continue his duties until a successor is chosen, or whether a temporary head of government may have to be appointed.
Mr Abe's decision on Wednesday triggered surprise and criticism.
He had defied calls for his resignation in the wake of the disastrous July polls and only two weeks ago reshuffled his Cabinet.
ROAD TO RESIGNATION
Sept 2006: Shinzo Abe is elected as PM, with long agenda of reforms
Early 2007: Series of scandals involving senior ministers
July 2007: LDP loses control of Japan's upper house for the first time in its history.
27 Aug: Abe reshuffles his Cabinet
9 Sept: Abe stakes his job on extending Japan's support of US-led mission in Afghanistan
12 Sept: Abe announces he is stepping down
But as well as his heatlh problems, Mr Abe said that his decision to step down was based on political realities resulting from the loss of the upper house.
The result left the opposition in a position to block key pieces of legislation, such as the extension of Japan's naval mission in support of US-led operations in Afghanistan.
"If I delay my decision to step down, the government will face greater difficulties in parliament," Mr Abe said.
'Ask the voters'
Taro Aso, a former foreign minister who is now the LDP secretary general, is seen as the strongest contender to take over from Mr Abe.
Another potential candidate is former Finance Minister Sadakazu Tanigaki, who ran against Mr Abe and Mr Aso for the top job in September 2006.
Other names mentioned include veteran lawmaker and former government spokesman Yasuo Fukuda and the current government spokesman, Kaoru Yosano.
The election of a party leader has been set for 23 September, but the LDP is under pressure to go further and call a snap election.
"With the LDP government thrown into this much confusion, the voters should be asked in the proper fashion who their choice for leader is in a general election," the Asahi newspaper said in an editorial.
"That is the only way to bring back politics based on the people's trust."