For developing an algebraic theory of functions on a locally compact groupoid, one needs an analogue of Haar measure on locally compact groups. This analogue is a system of measures, called Haar system, subject to suitable invariance and smoothness conditions called respectively left invariance and continuity. Unlike the case of locally compact group, Haar system on groupoid need not exists. In this paper we shall consider a locally compact groupoid G, and we shall denote by G(0) the set of units with singleton orbit s(0) and by Gs the reduction G|G(0) of G to Gs . We shall prove that if G admits Haar's systems, then the restriction of the range map at Gs is an open map from Gs to G(0) .Conversely, we shall prove that if this map is open at every x ∈ Gs , then the continuity condition of a Haar system holds at every unit with singleton orbit.