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4D CT is a dynamic volume imaging system of moving organs with an image quality comparable to conventional CT, and is realized with continuous and high-speed cone-beam CT. In order to realize 4D CT, we have developed a novel 2D detector on the basis of the present CT technology, and mounted it on the gantry frame of the state-of-the-art CT-scanner. In the present report we describe the design and the results of performance evaluation of the first model of 4D CT-scanner. The X-ray detector for the 4D CT-scanner is a discrete pixel detector in which pixel data are measured by an independent detector element. The numbers of elements are 912 (channels) × 256 (segments) and the element size is approximately 1 mm × 1 mm. The data sampling rate is 900 views(frames)/sec, and the dynamic range of the A/D converter is 16 bits. The rotation speed of the gantry is 1.0 sec/rotation. The data transfer system between rotating and stationary parts in the gantry consists of a laser diode and photodiode pairs, and achieves net transfer speed of 5 Gbps. Volume data of 512×512×256 voxels are reconstructed with FDK algorithm by parallel use of microprocessors. The image characteristics such as noise, uniformity and spatial resolution were evaluated with stationary phantoms in a single rotation. Exposure dose to an object was measured with an extension of the standard measurement method of the CT dose index (CTDI). Several volunteers were scanned to explore clinical potentials. For the 4D CT-scanner, the image characteristics of stationary objects and exposure dose were almost the same to that of conventional CT. An isotropic resolving power of less than 0.5 mm was achieved for the stationary object.