The division of a national territory is a mandatory process to analyze socio-economic dynamics. Com- muting is then an important dimension to build such classification and weighted network analysis is adapted to study this phenomena. We present in this paper a procedure to identify groups of cities where commuters flow are relatively dense through a case study: a huge network which represents commuting in France (based on the 1999 national census). Our approach is based on a common technique improved by visual tools: highlight dense areas using a strength metric and extract clusters using the variation of a quality measure function.