Someone is in charge of organizing a festival/meeting/conference etc. What do we call him? Can we say he is the administrator of that festival? What word suits better?
I would normally use "organizer" or "coordinator". I would tend to use "organizer" for larger events and "coordinator" for smaller events, but I wouldn't say it's a hard and fast rule.
"Administrator" as a title is usually reserved for either:
Heads of entities, especially those titled "administration". For example, the head of the BPA http://www.bpa.gov/news/AboutUs/Execs/Pages/Bill-Drummond.aspx. Note that the head of the US EPA (an Agency) is titled "Administrator of the EPA".
Positions involving overseeing and actively managing information technology objects, like computers, networks and files. In this case they are usually called "system administrators", "network administrators", "license administrators", and so on.
It is not incorrect to use the word administrator - the meaning is easily understood by readers - but godel9's suggestions are more common in English and sound more fluent, if that is important to you.