PALO ALTO, Calif.—The billionaire investor known as the Oracle of Omaha, Warren Buffett, is staging a second big fund-raiser to boost Senator Clinton’s presidential campaign.

The event, billed as a lunch and conversation with Mr. Buffett and Mrs. Clinton, is set to take place at a San Francisco hotel on December 11. Tickets will start at $100 for students and those under 30, and $250 for others.

In June, the Berkshire Hathaway chairman took part in a similar fund-raiser with Mrs. Clinton in Manhattan. That event reportedly raised about $1 million for her presidential bid.

However, Mr. Buffett has also offered praise for and money to one of Mrs. Clinton’s rivals for the Democratic nomination, Senator Obama of Illinois. The investing guru held a fund-raiser for Mr. Obama at an Omaha country club in August.

Mrs. Clinton, who was last in San Francisco about a week ago, has been passing through the left-leaning Bay Area about every four weeks to recharge her campaign coffers. Mr. Buffett’s lunchtime appearance may draw a business-oriented crowd that has escaped the senator’s previous fund-raising forays.

Mrs. Clinton has also set a fundraiser at a Los Angeles restaurant and bar, Social Hollywood, on December 10. However, that gathering is in some doubt since it revolves around a Democratic presidential candidates debate scheduled to take place in Los Angeles the same night. The debate is being organized by CBS News, whose news writers are threatening to strike. All the major Democratic candidates have said they will skip the debate and honor the picket lines if a strike is called.