iphone 7 2016

Tim Cook, chief executive officer of Apple Inc., unveils the new iPhone during an event in San Francisco on Sept. 7, 2016. 

David Paul Morris/Bloomberg via Getty Images

Warren Buffett says he's investing heavily in Apple, believing that once consumers begin using the company's products they aren't likely to stop.

The billionaire investor said on CNBC Monday that Berkshire Hathaway now holds about 133 million shares of Apple after buying about 120 million shares of the company's stock this year. That's more than double stake that Berkshire revealed earlier this month.

Buffett's aggressive buying spree at Apple caught many off guard because he has historically steered clear of technology companies. But Buffett says when he looks at Apple he sees a maker of consumer products, not technology. "Apple strikes me as having quite a sticky product," Buffett said.

While Buffett says Apple's iPhones are great, he still carries a basic flip phone.

Buffett appeared on CNBC after releasing his annual letter to Berkshire Hathaway shareholders on Saturday.