* Investors pile into France 30-year bond sale, books still open
* France's 10s/30s yield spread widens to highest since Dec 2014
* Germany, Belgium among potential sellers of long-dated bonds
* Euro zone periphery govt bond yields http://tmsnrt.rs/2ii2Bqr (Writes through)
LONDON, May 16 (Reuters) - France received more than 30 billion euros of orders for a new 30-year bond on Tuesday, highlighting strong demand for a sale seen as the first big test of sentiment following the French presidential election.
The gap between 10- and 30-year French government bond yields reached its widest since December 2014 ahead of the transaction, as investors made room for the supply by selling some of their existing holdings of long-dated French debt.
France's debt agency has been monitoring the market for a 30-year bond sale via syndication -- where the borrower appoints banks to sell bonds to investors directly rather than via an auction -- and started marketing the deal on Tuesday.
Although French government bonds have rallied sharply since it became clear that centrist Emmanuel Macron would beat far-right leader Marine Le Pen to the presidency, Tuesday's sale is seen as a barometer of investor sentiment towards France.
"The appetite for this deal should be good but we will be watching it closely because parliamentary elections are coming up and they will determine whether Macron can form an effective government," said ING strategist Benjamin Schroeder.
Macron appointed a conservative prime minister on Monday in a move to broaden his political appeal and weaken his opponents before parliamentary elections in June.
Some will be watching the deal to see if there's any sign of a return of Japanese investors, key lenders to France.
"Japanese investors have been large buyers of French securities and given that the political risk has reduced, we should see a gradual return to this market," said Antoine Bouvet, rates strategist at Mizuho.
France opened books on the deal around 0830 GMT on Tuesday, having recorded interest in excess of 18 billion euros even before it started taking orders, and by mid-morning it had received 30 billion euros of orders.
The bond matures in May 2048, three years later than the current 30-year benchmark but still eligible for ECB purchases in June, according to Societe Generale.
Tuesday's sale will also demonstrate investor appetite for duration, particularly with other long-dated bond sales expected from Germany, euro zone bailout fund EFSF and possibly Belgium.
Mizuho analysts said in a note that the spread between 10- and 30-year euro zone debt has been widening since the middle of last month in anticipation of further issuance.
Germany is slated to sell 30-year bonds on Wednesday while Belgium has cancelled a bond auction for next week. Analysts said that may be because the Belgian debt agency is planning a 15- or 20-year bond sale this week.
The European Financial Stability Facility, the euro zone's bailout fund, is widely expected to sell long-dated bonds this week, while Slovenia was selling bonds maturing in 2027 and 2040 on Tuesday.
The gap between France's 10- and 30-year debt rose to 108 basis points on Tuesday, the highest level since December 2014.
High-rated euro zone bond yields were flat to a basis point higher as euro zone economic output numbers for the first quarter of the year were strong, but in line with expectations.
The yield on Germany's 10-year government bond, the benchmark for the region, was up 1.6 basis point to 0.435 percent.
Lower-rated Southern European bonds outperformed, with the yield on Spanish, Italian and Portuguese 10-year government bonds down 3-4 bps.
Bonds from the bloc's periphery tend to perform well on signs the euro zone economy is strengthening.
German Chancellor Angela Merkel and French president Macron agreed on Monday to draw up a roadmap to deeper European Union integration and opened the door to changing the bloc's treaties to facilitate ambitious reform.
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(Addition reporting by Dhara Ranasighe; Editing by Catherine Evans)