PURPOSE: To obtain a high tracking accuracy, by a method wherein a tracking locus obtained in the last day of a heliostatic operation is constantly renewed, and is fed to a driving system for a heliostat as a controlling output signal when a controlling input signal is interrupted.

CONSTITUTION: When the weather is fine, the titled device is driven by closed loop control, and data of time (t) and coordinates X, Y for one day are sotred into a memory at appropriate time intervals. When initial driving of the device is conducted when the weather is fine all day long, a one-day amount of data are completely obtained for each machine base. When input of rays to a sensor is interrupted, coordinates data Xi, Yi at time ti are retrieved from the memory at regular time intervals, and are directly outputted to the driving system as controlling signals.