This paper presents a new technique for embedding information inside a 3-D printed object. The information is added to the 3-D model data, and when the 3-D printed object is fabricated using the data, the dispositions of fine domains, which express the information, are simultaneously formed inside. In this study, fine cavities are formed as fine domains, and the thermal conductivity of the cavity is lower than the main body of the object filled with the material. Therefore, the disposition of the cavities can be detected from the surface temperature of the object. Experiments were conducted using a fused deposition-modeling 3-D printer and thermography, and their results revealed cavities as small as 2 × 2 mm in size could be detected even when the surface was curved. We demonstrated the proposed technique was feasible.