German authorities had noted price differences that ranged from 25 to 45 percent, a statement said. The cartel office would therefore check whether recent German gas price hikes were justified, it added.
Head regulator Bernhard Heitzer said preliminary results of the probe suggested that competition was "still horribly weak" in the gas sector. The price increases concerned four million clients and around 20 percent of the market, the office said.
A spokesman contacted by AFP declined to provide the names of companies under investigation, saying only that national and regional firms were concerned.
In all, 770 companies supply gas to households and companies in Germany, of which around 30 are active throughout the country.
Regional anti-cartel offices had also launched their own investigations into gas price increases, the federal office spokesman said.