Instead of create some administration pages, I want that my customers, can change only the content of the page (in admin). I explain :) :
Can I create a user who can access of the content section (in admin mode) on only, one or some pages ? . So he can use the wysiwyg editor, but only this, on a page.
Yes, that should work. Just create a new Group in the Security section and add the required permissions (SiteContent and maybe Files & Images). Then create a User in that group (this would be your customer login). Each Page can have its edit rights restricted to certain groups. Just click on the page you'd like to modify and then open the "Access" tab. If the customer should not be able to modify the page, grant the edit rights just to the Administrator group.