This paper proposes a sensor system to identify mobile robot's current position by measuring target direction angles. Basic component of the sensor system is constructed with a revolutionary sensor head to detect a landmark. The sensor head tracks the direction of landmark using a characteristic of directivity for sensor unit. Two of the component are combined to measure horizontal and vertical angles corresponding to the direction of the landmark. Using two measured values by the sensor system, mobile robot's current position is calculated by a geometrical relation of the mobile robot and the landmark. An experimental target direction sensor system for the mobile robot is developed. Basic characteristics of the developed sensor system is shown in this paper.