The nomination of Social Democrat Schroeder to the board of Rosneft, subject to Western sanctions over Moscow's role in the Ukraine crisis, has caused an outcry in Germany, especially in a climate of fear about any potential Russian interference in the Sept. 24 vote.
"I do not think what Mr. Schroeder is doing is okay," Merkel, a conservative, told Bild newspaper in an interview broadcast live online, Reuters reported.
"I don't intend to take any posts in industry once I am no longer chancellor, but I am first fully concentrated on the election and the fact that I would like to be chancellor again," added Merkel, 63.
Combative Schroeder, 73, who makes no bones about his friendship with Russian President Vladimir Putin, has defended his decision to join Rosneft and accused his foes of political maneuvering to help get Merkel re-elected.
Last week, he said his planned new job at Rosneft would not damage the Social Democratic Party (SPD).
The SPD are trailing Merkel's conservatives in most polls by around 15 percentage points though many commentators expect them to again join a right-left "grand coalition".
Schroeder is already a divisive figure in the SPD due to his labor market reforms and some members have distanced themselves from their former leader.
Russia owns 50 percent plus one share of Rosneft and some conservatives and Greens have also criticized Schroeder for his "shameless" behavior.
Schroeder is already chairman of the shareholders' committee of Nord Stream AG, a Gazprom-led consortium established for the construction of a pipeline carrying Russian natural gas across the Baltic.
It is not the first time Schroeder has been attacked for his ties to Russia. Just weeks after Russia annexed Crimea in 2014; he drew criticism for giving Putin a bear hug at the Russian leader's birthday party in St Petersburg.
Schroeder, whose father died in World War Two fighting the Red Army, is also still ridiculed for saying back in 2005 that he agreed with a description of Putin as a "flawless democrat".