A planar millimeter-wave 2-D beam-scanning multibeam array antenna fed by compact 16-way beam-forming network (BFN) in multilayered substrate integrated waveguide (SIW) technology is addressed. The BFN is formed by connecting two stacks of sub-BFNs, the E-plane sub-BFN and the H-plane sub-BFN. The H-plane sub-BFN is realized by a traditional H-plane <inline-formula> <tex-math notation="LaTeX">4 \times 4 </tex-math></inline-formula> Butler matrix (BM). The key point of this design is to propose an E-plane <inline-formula> <tex-math notation="LaTeX">4 \times 4 </tex-math></inline-formula> BM which realizes a planar E-plane sub-BFN. These two sets of sub-BFNs can joint directly without resorting to any connectors or connecting networks to form such a compact 16-way BFN with a reduced area of merely <inline-formula> <tex-math notation="LaTeX">3\lambda \times 12\lambda </tex-math></inline-formula>. After that, to be compatible with the proposed BFN, a ladder-type <inline-formula> <tex-math notation="LaTeX">4 \times 4 </tex-math></inline-formula> slot antenna array is employed, which is comprised of four linear <inline-formula> <tex-math notation="LaTeX">1 \times 4 </tex-math></inline-formula> slot antenna arrays. Different from traditional array, the four subarrays are distributed in separate layers for the purpose of jointing to the BFN more conveniently. Transition network are also required to connect the BFN with the antenna array. Finally, a compact 2-D scanning multibeam array antenna based on the planar SIW BFN are fabricated and measured, which would be an attractive candidate for 5G application.