To secure a high-precision tracing function even if the intensity distribution of receive light becomes ununiform owing to perturbations in the air.

An angle error detection optical system 30 is driven and set with a movable mechanism 29 so that the spot diameter of a received beam on an optical detector is nearly equal to the minimum resolution size of the optical detector. The X-axial received power difference at this time is (PX+-PX-) and the Y-axial received power difference is (PY+-Py-). Even when the intensity distribution of the received beam varies owing to perturbations in the air, the received power differences do not vary because of the spot diameter equal to the minimum resolution of the optical detector. Transmitted light can, therefore, be sent in the arriving direction of the received beam without tracking deviation.