We study a filtered vector space A d (n) over a field k of characteristic 0, which consists of Jacobi diagrams of degree d on n oriented arcs for each n, d ≥ 0. We consider an action of the automorphism group Aut(Fn) of the free group Fn of rank n on the space A d (n), which is induced by an action of handlebody groups on bottom tangles. The action of Aut(Fn) on A d (n) induces an action of the general linear group GL(n, k) on the associated graded vector space of A d (n), which is regarded as the vector space B d (n) consisting of open Jacobi diagrams. Moreover, the Aut(Fn)-action on A d (n) induces an action on B d (n) of the associated graded Lie algebra gr(IA(n)) of the IA-automorphism group IA(n) of Fn with respect to its lower central series. We use an irreducible decomposition of B d (n) and computation of the gr(IA(n))-action on B d (n) to study the Aut(Fn)-module structure of A d (n). In particular, we consider the case where d = 2 in detail and give an indecomposable decomposition of A 2 (n). We also consider a functor A d , which includes the Aut(Fn)-module structure of A d (n) for all n.Contents 30 References 31