A minimal model for multipole orders in CeB 6 shows that degeneracy of the quadrupole order parameters and strong spin-orbit coupling lead to peculiar temperature and magnetic-field dependences of the X-ray reflection intensity at superlattice Bragg points. Furthermore, the intensity depends sensitively on the surface direction. These theoretical results explain naturally recent X-ray experiments in phases II and III of CeB 6 . It is predicted that under weak magnetic field perpendicular to the (111) surface, the reflection intensity should change non-monotonically as a function of temperature.KEYWORDS: X-ray scattering, CeB 6 , quadrupole order, multipole orderIn CeB 6 and related systems, presence of both orbital and magnetic degrees of freedom brings about rich structures in the phase diagram. In zero field, CeB 6 turns into the antiferro quadrupole (AFQ) ordered phase (phase II) at T Q = 3.4K and turns into antiferromagnetic (AFM) ordered phase (phase III) at T N = 2.3K.1 The order parameter in phase II is the Γ 5g -type quadrupole moment. In phase III, the non-collinear magnetic structure is described by four wave numbers:Recently, X-ray scattering has been utilized as a powerful probe to detect orbital orderings.2 For CeB 6 , Yakhou et al. have performed resonant and non-resonant scattering experiments and found superlattice reflections in phases II and III. 4 More recently, Tanaka et al. have reported unexpected temperature and magnetic field dependences using non-resonant X-ray scattering. 5,6 Although the superlattice spots emerge below T Q , the intensity of (n/2, n/2, n/2) reflections with n odd integers remains small in T N < T < T Q . The intensity increases almost stepwise below T N . 5,6 Furthermore the small intensity in phase II is suppressed by application of a magnetic field of as small as 0.1 T, while the suppression in phase III requires an order of magnitude larger magnetic field.6 These features seem strange at first sight since the staggered quadrupole moments probed by X-rays should already be present in phase II, and should not change significantly below the Néel temperature T N . In this paper we demonstrate how X-rays probe the coupling among dipole, quadrupole and octupole moments, and show that consideration of a quasi-continuous symmetry of the quadrupole order parameters provides a natural explanation of the experimental observations.In CeB 6 , a localized 4f electron of a Ce 3+ ion has the quartet crystalline electric field (CEF) ground state, which is called Γ 8 and is well below the excited CEF level Γ 7 .7 An orbital pair in the Γ 8 level are given by are written as |± ↓ , and are obtained by reversing the sign of J z in eq. (1). To describe multipole operators, we introduce two kinds of pseudo spin operators σ and τ as τ z |± ↑ = ± |± ↑ , τ z |± ↓ = ± |± ↓ ,Then the magnetic moment M is given by