Warren Buffett remains bullish on index fund investing.

After all, his own company can't beat them.

The billionaire investor told CNBC on Monday that Berkshire Hathaway's chief stock pickers, Ted Weschler and Todd Combs, have failed to beat the .

"Overall, they are a tiny bit behind the S&P, each, by almost the same margin," Buffett told Becky Quick on "Squawk Box" from Omaha, Nebraska.

Buffett has long championed low-cost index funds.

"I think it's the thing that makes the most sense practically all of the time," he recently told CNBC's On The Money.

Index funds are a form of passive investing, and they hold every stock in an index. The S&P 500, for example, owns big-name companies, including Apple, Microsoft and Google.

Buffett told CNBC's Squawk Box recently that if someone invested $10,000 in an index fund back in 1942, it would be worth $51 million today.