Iran leader faces Holocaust case in Israel
Mr Ahmadinejad was widely criticised when he said last year that the Holocaust was a "myth" and that Israel should be "wiped off the map".
Germany passed a law in 1993 forbidding Holocaust denial. It is punishable by up to five years in prison.
Six million Jews were killed by the Nazis during World War II.
Mr Shahar said he wanted the German federal prosecutors' office to take the issue before the constitutional court in the hope that international arrest warrants would be issued against Mr Ahmadinejad.
Correspondents say prosecutors will have to consider whether Germany has jurisdiction and whether President Ahmadinejad enjoys immunity.
"I'm awaiting a response about whether they will file charges but I don't know how long it will take," Mr Shahar told the Reuters news agency.
"It doesn't take days but several months."
On Monday, British historian David Irving was found guilty in Austria of denying the Holocaust and sentenced to three years in prison.
He had pleaded guilty to the charge, based on a speech and interview he gave in Austria in 1989.
Although Mr Ahmadinejad did not deny the Holocaust on German soil, another law passed in 2005 permits the filing of international cases in German courts.
Mr Shahar hopes the case might result in international warrants for Mr Ahmadinejad's arrest, thus preventing the president from entering the US or Europe.
The lawyer believes Germany is the best country in which to file the suit, because it is the "most influential European state" and because the country is directly linked to the case.
The prosecutors' office in Karlsruhe has not yet commented.