Since a large-scale in-memory data store, such as key-value store (KVS), is an important software platform for data centers, this paper focuses on an FPGA-based custom hardware to further improve the efficiency of KVS. Although such FPGA-based KVS accelerators have been studied and shown a high performance per Watt compared to software-based processing, since their cache capacity is strictly limited by the DRAMs implemented on FPGA boards, their application domain is also limited. To address this issue, in this paper, we propose a multilevel NOSQL cache architecture that utilizes both the FPGA-based hardware cache and an in-kernel software cache in a complementary style. They are referred as L1 and L2 NOSQL caches, respectively. The proposed multilevel NOSQL cache architecture motivates us to explore various design options, such as cache write and inclusion policies between L1 and L2 NOSQL caches. We implemented a prototype system of the proposed multilevel NOSQL cache using NetFPGA-10G board and Linux Netfilter framework. Based on the prototype implementation, we explore the various design options for the multilevel NOSQL caches. Simulation results show that our multilevel NOSQL cache design reduces the cache miss ratio and improves the throughput compared to the non-hierarchical design.