Guardian Media Group has appointed Andrew Miller, the chief finance officer of Trader Media Group, as group finance director.

Miller will replace Nick Castro, who is to retire in the autumn after 11 years at GMG.

He joined Trader Media Group, the publisher of Auto Trader magazine – which is jointly owned by GMG and private equity group Apax Partners – in 2002 as group finance controller.

Miller was appointed to the board of TMG as chief finance officer and played a major role in the sale of 49.9% of the business, previously wholly owned by GMG, to Apax in 2007.

Apax and GMG, publisher of the Guardian newspaper and MediaGuardian.co.uk, have begun the search for a replacement for Miller at TMG.

Castro, who played a key role in the TMG sale and the £1bn joint acquisition, again with Apax, of Emap's business-to-business magazine and events arm, will retire from GMG in the autumn.

"Nick has been an exemplary finance director for GMG, and his contribution to the expansion and achievements of the group in recent years has been immense," said the GMG chief executive, Carolyn McCall.

"We are very sorry to lose him, but in Andrew Miller we have a first-class successor. Andrew has been a key player in Trader Media Group's phenomenal success and he also has a deep understanding of the unique nature of GMG."

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