The demand for non-destructive inspection of the thermal insulation layer of Japanese houses has been increasing. Surface temperature measurement is commonly used for estimating the condition of the thermal insulation layer that is located inside the walls. However, the accuracy needs to be improved because this approach only considers the surface information. To evaluate the thermal insulation layer inside the walls, a millimeter-wave tomography system is proposed for measuring the water content. The system can provide ∼10 mm range resolution to differentiate the reflections from the thermal insulation layer behind the external wall. The Lichtenecker-Rother model is applied for the quantitative evaluation of the water content using the reflected signal. The proposed model is consistent with the experimental data, confirming that a maximum error of 16.0% is obtained. It is also demonstrated that the water content distribution can be visualized with a range resolution of 10.6 mm.