ok done, the 404 custom page is correctly displayed but I can' disable the component heading's mention "Article" just above the diplayed title "404" (which I can disable in editing the article in the joomla artcile management). How can I fix it ?
Thanks !

JoomSEF's custom 404 page is stored as a standard uncategorized Joomla article titled 404. So just open Articles manager in Joomla and find the 404 article. There you can set any options for it you'd like.

The problem is I have 2 titles :
- 1st : a "componentheading" class title wich is not manageable is the article management => I would hide it
- 2d : normal title (404) of the article that I can manage normally
I specify that this first title is not a "categorie title" or a "parent categorie title" you can manage in the article.
The problem is specific to this page, is it linked to joomsef ? Should I delete something in the core?
Thanks

[SOLVED]

Ok, I think this topic is solved : I redirect 404 on homepage, so no matter with the 404 page display.

Thanks