This paper presents a novel gait analysis system that is easy to use by patients and doctors. The system uses a new gait analysis device called “CAterpillar Type trAnsParent treadmill (CaTTaP)” and an image processing method. The CaTTaP has a “caterpillar” type of walking surface made of acrylic resin plates. A camera embedded in the CaTTaP walking surface continuously captures images of the sole during walking. Image processing of the plantar images extracts the plantar regions and determines the position of the subject’s sole on the CaTTaP. To evaluate the usefulness of the proposed method, we calculated the step length and width. For this study, we recruited 6 robust elderly persons. We conducted measurements with each subject walking at 3 different velocities. A total of 302 step lengths and widths were calculated from the image data, and the accuracy of the results were estimated by comparing with measurements obtained simultaneously using a motion capture system. Analysis of the absolute values of differences showed that the accuracy of the proposed system varied among subjects. In the higher accuracy group, step length and width were estimated with absolute differences of approximately 16.1 mm and 31.2 mm, respectively, and the entire sole was stably and steadily in ground contact from the heel to the toe. However, in the lower accuracy group, step length and width were estimated with absolute differences of approximately 61.2 mm and 34.4 mm, respectively, and the toes were sometimes off ground. The results suggest that the accuracy of the proposed system varies depending on the type of gait, and that the image processing method requires further improvement. However, the proposed system provides a new approach to gait analysis that requires a short walk and no preparations or special wear. In addition, the CaTTaP has the potential to measure parameters such as the plantar contact area, plantar skin deformation, and shape of the sole with clearer dynamic plantar images that are difficult to obtain continuously by other devices. The novel CaTTaP is one of the best gait analysis systems available for clinical use.