Investment pioneer Jack Bogle says stocks are 'relatively expensive' but not compared to bonds

• "My own opinion is the market is pretty fully valued here," says the Vanguard founder.
• Compared to bonds, stocks look "a lot less expensive," Bogle says.
• Bogle did express concern about which measure of earnings investors should be paying attention to.

The overall stock market as measured by the has soared just more than 10 percent since Election Day and prices are looking "pretty fully valued" at current levels, Vanguard founder told CNBC on Monday.

"My own opinion is the market is pretty fully valued here. [But] I don't think in a highly risky way," Bogle said on "Squawk on the Street."

"When you compare them to bonds they look a lot less expensive," he argued. "Stocks will almost certainly do better than bonds."

But Bogle did express concern about which measure of corporate earnings investors should be paying attention to.

"I think the important thing to watch is this big gap that we are seeing right now in operating earnings in the future compared to reported earnings in the past like GAAP earnings, generally accepted accounting principles," he said.

"If you look at GAAP earnings backwards, what actually has been done, the P/E is about 26. And if you look at future earnings ... only operating earnings without all the bad stuff in them, the P/E drops down to 16."

"So is it 26 or is it 16? Or is it something in between?" he asked, though he did not provide an answer beyond saying that measuring the market by its price-earnings ratio shows it's "relatively expensive."

Bogle founded Valley Forge, Pennsylvania-based Vanguard in 1975. The fund group has $4 trillion in global assets under management, with more than 20 million investors in about 170 countries.