For the energy-resolved computed tomography (CT), the authors developed a current mode X-ray detector, a “transXend detector”, which measures X-rays as electric current and gives X-ray energy distribution after analysis. As an application of the transXend detector, thickness distributions of materials are estimated with one transmission measurement, with using two-dimensional map made of the ratio of measured electric currents. With two transmission measurements, a tomography is reconstructed. Large reduction of dose exposure in CT is expected with this method.