* Big manufacturers' sentiment index at plus 25 vs f'cast
* Service-sector sentiment at plus 23, matches f'cast
* Firms expect conditions worsen 3 months ahead - tankan
* Big firms expect capex to rise 7.4 pct vs f'cast +7.5 pct (Adds analyst quote)
TOKYO, Dec 15 (Reuters) - Big Japanese manufacturers' business confidence improved for a fifth straight quarter in the three months to December to hit a 11-year high, a central bank survey showed, a sign the economy is gathering momentum from robust exports and booming corporate profits.
The data may help the Bank of Japan (BOJ) make the case that strengthening economic recovery will prompt firms to raise wages and allow it to edge away from crisis-mode stimulus, even before inflation hits the central bank's elusive 2 percent target.
The headline index for big manufacturers' sentiment stood at plus 25 in December, the BOJ's closely watched "tankan" survey showed on Friday, up from plus 22 in September and slightly higher than a median market forecast for plus 24.
It matched a high marked in December 2006.
An index measuring big non-manufacturers' sentiment was unchanged from September at plus 23, matching a median market forecast.
"The tankan results support the BOJ's bullish economic view backed by global economic recovery, although it suggests weakness in private consumption with sentiment worsening in sectors like restaurants, hotels and other services," said Hidenobu Tokuda, senior economist at Mizuho Research Institute.
"Capital expenditure remains firm due to investment to cope with labour shortages, the 2020 Tokyo Olympics, urban redevelopment and distribution network. This is a long-term trend that should support corporate capex into the coming year."
Big manufacturers and non-manufacturers expected business conditions would worsen in the next three months, the poll found.
Big firms expect to increase capital expenditure for the current fiscal year to March 2018 by 7.4 percent, compared with a median market forecast for a 7.5 percent gain, it showed.
Japan's economy expanded an annualised 2.5 percent in the July-September period to mark a seventh straight quarter of expansion, supporting the central bank's recent signals that it could move away from crisis-era monetary policy.
But many analysts expect core consumer inflation, now at 0.8 percent, to slow next year unless firms pay heed to Prime Minister Shinzo Abe's calls to hike wages by 3 percent - no easy task given how wary they have been about raising salaries so far.
The tankan will be among key data the BOJ will scrutinise at its next rate review on Dec. 20-21.
The tankan's sentiment indexes are derived by subtracting the number of respondents who say conditions are poor from those who say they are good. A positive reading means optimists outnumber pessimists. (Reporting by Leika Kihara; Editing by Eric Meijer)