PURPOSE: To reduce power consumption at time of tracking, and to lower the price of a device, by constituting the device so that parting lines in which a photodiode array is divided equally, are formed so as to be crossed only at one intersection, and the tracking is performed by moving an array and a light emitting element integrally.

CONSTITUTION: An emitting beam of light from a fixed station 1 is converged by a lens 2, and is radiated on the photodiode array 21. The array 21 is quartered so as to have only one center, and is formed with clements 21a, 21b, 21c, and 21d having the same sizes. Arithmetic circuits 251∼257 generate simultaneously a tracking signal and a main signal which integrate the array 21 and the light emitting element 8 so as to coincide the center of a light flux with the center of the array 21, and drive the same plane in a biaxial direction, based on the output of each element. The tracking is performed in such way by moving the array 21 and the light emitting element 6 in the flat plane integrally. Therefore, inconvenience to turn the whole of a transmission/reception optical system in a mobile station at time of tracking can be dissolved.