In recent years, products consisting of multi-materials have attracted attention due to their potential for drastically increasing product performances and creating new physical characteristics. To facilitate obtaining designs based on the use of multi-materials, a systematic optimal design method especially adapted for use with multi-materials is required. To this end, this paper presents a topology optimization method for multi-materials in which two different materials are represented in the design domain, in addition to a void phase. We apply the Multi-Material Level Set (MM-LS) topology description model, which uses a total number of n level set functions to provide a representation for n materials, and the void phase. The advantage of the MM-LS model is that clear optimal configurations are obtained and the design sensitivities for multi-material structures can be easily calculated. A reaction diffusion equation is solved to update the distribution of the level set functions, which enables control of the geometric complexity of the optimal configuration. This paper is organized as follows. Following a brief introduction, the proposed level-set based multi-material topology optimization method is discussed first. Next, a stiffness maximization problem for a multi-material structure is formulated and we derive the design sensitivities based on the adjoint variable method. Finally, several numerical examples are provided to confirm the validity of the proposed method.