To save people opening the link...in France it would be a judge not a prosecutor. France has an Inquisitorial rather than the Adversarial legal system the UK and US have. Put simply, a judge doesn't merely decide between the two cases presented to them, they try and establish the facts

Edit: I said 'UK' where I should have said 'England and Wales'. Scotland and Northern Ireland have their own legal systems, although I believe both have Adversarial systems they are different in some ways. The US system could, however, be seen as a continuation of the English system.