There are two approaches to three-dimensional (3D) image reconstruction using a 1D array ultrasonic transducer: mechanical linear scanning and free-hand scanning. Mechanical scanning employs a motorized mechanism to translate the transducer linearly. However, the large size and weight of the scanning system sometimes make it inconvenient to use. In free-hand scanning, a sensor (e.g., electromagnetic or optical) is attached to the ultrasonic transducer to measure the position and orientation of the transducer. These techniques are sensitive to the usage environment. Recently, sensorless free-hand scanning techniques have been developed. Seabra et al. reported sensorless free-hand techniques for the carotid artery by monitoring the velocity of the ultrasound probe [J. C. R. Seabra, L. M. Pedro, and J. F. Ferandes: IEEE Trans. Biomed. Eng. 56 (2009) 1442]. This system achieved an accuracy of 2.5 mm [root mean square (RMS) error] of the location. To develop accurate sensorless measurement, we propose a novel method using the phase shift between ultrasonic RF echoes. In this study, we measured the transmit-receive directivity of a linear-array transducer using a silicone phantom and estimated the elevational distance between two 2D US images using the phase shift. An accuracy of 49.9μm in RMS, which is less than that of the previous sensorless free-hand method, could be achieved by the proposed method.