* Offer is for 4.2 euros per share
* Majority-owner Casino supports offer
* Would be Dutch No.2 behind Ahold's Albert Heijn
* Boosts Casino's plans to raise money to pay off debt
* Super de Boer shares jump 19 percent, Casino up 2 percent
(Adds more comment by shareholders, updates shares)
By Reed Stevenson and Mark Potter
AMSTERDAM/LONDON, Sept 18 (Reuters) - Dutch grocer Jumbo has made a 480 million euro ($706 million) bid for bigger rival Super de Boer, the two said on Friday, in a move to boost the market power of the country's second-biggest food retailer.
A deal would step up competition against Dutch market leader Albert Heijn, owned by Ahold, and would be a boost for French grocer Casino, which owns 57 percent of Super de Boer and is looking to raise money to pay off debt.
"The takeover will change the landscape," said Petercam analysts. "The most successful banner of the last five years will more than double in size with an estimated market share in excess of 11 percent."
Analysts said Jumbo was paying a high price, which was likely to deter a counterbidder.
Its 4.2 euros a share offer is 20 percent above Super de Boer's closing price of 3.49 euros on Thursday. Super de Boer;s share price had already climbed from about 2.8 euros at the start of the month on speculation of an offer.
The price was good news for Casino, which is looking to raise 1 billion euros from selling non-core assets by the end of 2010, analysts said.
"When Super de Boer became fully consolidated in 2008 Casino's group operating margin was negatively impacted by 22 basis points. The sale of the business should therefore be beneficial to margins," Barclays Capital analysts said.
Casino, which runs more than 10,000 stores in 10 countries, said it supported the offer and it would get around 300 million euros for its stake -- higher than the value on its balance sheet.
However, another Super de Boer shareholder, Jaap Bastmeijer who owns at least 5 percent of the business, described Jumbo's offer as "very weak."
"Jumbo and Super de Boer will have to sit down with us to discuss the offer," he said.
At 1140 GMT Super de Boer shares were up 18.6 percent at 4.13 euros. Casino was up 1.9 percent at 56.5 euros and Ahold, which has about 30 percent of the Dutch grocery market, up 0.4 percent at 8.39 euros.
Petercam analysts said competition regulators might have concerns in some localities, which could lead to the sale of a few stores following the deal.
Super de Boer, already the Netherlands' second-biggest food retailer, completed a restructuring in 2007 by selling loss-making stores and streamlining its distribution and logistics facilities.
Earlier this week Super de Boer reported flat sales of 470 million euros for the third quarter, slightly below analysts' expectations. ($1=.6802 euros) (Additional reporting by Helen Massy-Beresford in Paris and Gilbert Kreijger in Amsterdam; Editing by Mike Nesbit and Greg Mahlich)