Just select the sub-project in the project filter (a sub-project is a project).

We don't see the administrative projects in the list

You should see them only if you are affected to them.

Sorry, it's not for sub-project but for activities
How can I obtain the time spent by user for just one activitie ?

I didn't find report with this possibility

Yannick

Several reports will help you :
- Work => Detailed work per resource
- Planning => Monthly planning resource/project/activity

Or easier : just access the activity.
You'll have real work in the assignment.