FRANKFURT, Oct 1 (Reuters) - Mario Draghi's chief adviser is quitting the European Central Bank to take up a post at French insurer AXA, leaving the ECB president without an ally who helped shape his plans to fight the euro zone crisis.

Christian Thimann, a 47-year-old German, quickly became a trusted adviser or "consigliere" to Draghi after the Italian, viewed with suspicion by many in inflation-wary Germany, took over the ECB presidency on Nov. 1, 2011.

A softly spoken ECB veteran, who has been at the bank since it was founded in 1998, Thimann's experience and understanding of Germany have proven highly valuable to Draghi, who used him to help sharpen views and plans, and also promoted him.

Sharing adjacent offices on the 35th floor of the Eurotower, the two men met regularly from the start of Draghi's presidency, brainstorming before Draghi crystalised his "fiscal compact" plan - a pitch for stricter budget discipline that preceded his signature policy measure, the OMT bond-buy programme.

Thimann, whose wife is French, will join AXA on Jan. 1, 2014, as group head of strategy and public affairs. The ECB gave no reason for his departure.

Draghi thanked Thimann for his "outstanding contribution to our work". The ECB did not immediately announce a successor.