Berlin prosecution office spokesman Martin Stelt said Ernst was being investigated for embezzlement and fraud. He was suspected of abusing his MPs' travel allowance by claiming expenses on trips to attend union meetings and company boards meetings between 2007 and 2009.
The allowance can be claimed only for activities directly related to being an MP. Stelt told daily Die Welt the investigation had been running for two weeks.
But Ernst told the paper he was not too concerned about the investigation.
“I am confident that I've conducted myself appropriately,” he said.
Should the allegations be proven, Ernst would face a fine or prison of up to five years.
The revelation comes hot on the heels of the federal court decision on Wednesday that Germany's domestic intelligence services could continue keeping tabs on members the Left party.
That decision overturned a ruling by a state court in North Rhine-Westphalia, which had said it was not appropriate for Germany's intelligence agency the Verfassungsschutz to be gathering a file on The Left's Thuringia state party leader Bodo Ramelow.
Domestic intelligence officials consider parts of The Left's agenda go against Germany's constitution, and the ruling by the federal court in Leipzig is a major setback for the far-left party's efforts to stop the government surveillance of its members.