Network slicing is a technology that logically splits a shared network infrastructure into multiple virtual networks. Each virtual network (called network slice) is allowed to have a different amount of virtualized network resources such as link bandwidth, buffer capacity, and computational functions. Due to limited network resources, virtual resources of the slices should be appropriately managed to satisfy diverse quality-of-service (QoS) requirements of users and services. In this paper, we focus on buffer resource management of network slices with different QoS requirements. Depending on traffic characteristics and QoS requirements of network slices, the sensitivities of changes in buffer size of network slices to QoS performance would be different. We propose a buffer-sizing scheme based on the QoS sensitivities of buffer allocation in network slices with various QoS requirements. Our experimental results using a Mininet emulator show that the proposed scheme could achieve a buffer allocation that maximizes the QoS satisfaction.