Dedicated to professor Joji Kajiwara on his 60th birthdyi.e. a system of rationally holonomic differential equations [3], [21].The purpose of the present paper is to study the (ir)reducibility conditions of the Gauss-Manin system associated with the integral above. Here the (ir)reducibility of a system means the (ir)reducibility of its monodromy representation.Throughout our arguments, we adopt a framework of the theory of twisted rational de Rham cohomology and twisted homology, which will be briefly reviewed in Section 1. Let the many-valued n-ίorm Φdt γ .. .dt n be the integrand of (0.1). A basis of the twisted n-th de Rham cohomology group attached to Φ naturally induces a system of matrix valued differential equations of the first order, which is called the Gauss-Manin system. Our first result is its explicit expression (Proposition 2.1). Luckily enough, our system happens to have logarithmic poles with residues of constant matrices, which makes our discussion much simpler. A study about (ir)reducibility is made in Section 3. Our second result is the irreducibility condition when v = 0. In this case, it is shown that if λj (1 < j < m) <£ Z and Σjli λj £7A then our system is irreducible (Theorem 3.1). This suggests thatReceived May 7, 1993. our Gauss-Manin system is generically irreducible, though we could not succeed in finding (ir)reducibility conditions in general setting. If a given system of differential equations has a subsystem, then the monodromy representation of the original one is reducible, of course. It seems an interesting problem to find a subsystem of our system. We find four reducible cases, when m -3 (Theorem 3.3). We also prove that the Lie algebra generated by the residue matrices is isomorphic to the general linear Lie algebra Qi(n + 1 C) (Proposition 3.4). The author believes that this Lie algebra should shed light on the irreducibility conditions in the future.When m is arbitrary, we obtain a reducibility condition and a corresponding subsystem in Theorem 3.5: We find that if 0 < £ < n -1 and 2λ } ,+ v£ = 0 (j = 1,... ,rn -2) then our system is reducible. By virtue of the formula in Theorem 3.5 in the case £ -1, we can derive a system of the differential equations of the second order which is satisfied by the integral (0.1) (Theorem 4.1). This system is related with the spherical functions of BC type defined in [11].We finally give a comment that the integral (0. is generated by the symmetrization of the logarithmic forms(2) If we suppose Π \λ m + -w k) ^ 0, in addition to the assumption (Mon),then a basis of H (12* (*/)), V ω ) n is given by the symmetrization ofThis theorem was first proved by Aomoto [3]. His proof depends on the arguments for the generalized Pochhammer differential equations. On the other hand, Esnault-Schechtman-Viehweg gave another proof of (1) Let S ω be the local system on X defined by the monodromy of Φ and S ω the dual local system of S ω . In this paper we consider only the (S n -) symmetric (co)homology, i.e., the (twisted) cycles Γ in the integral (0.1) ar...