A main controller 211 controls a fiber-end drive mechanism 140 and an attenuator 121 with reference to the received-light amount of interference light LC to cause the following operations (1) and (2) to be executed alternately: (1) moving an emission end 116 to increase the received-light amount to at least an upper limit; and (2) changing the light amount of reference light LR to decrease the received-light amount to at least a lower limit. When the received-light amount specified by an received-light-amount specifying part 212 decreases in response to the movement of the emission end 116 in (1), the main controller 211 controls the fiber-end drive mechanism 140 to return the relative position to the immediately preceding status of this change. The main controller 211 leads the received-light amount of interference light LC by a CCD image sensor 120 to a target value by controlling the attenuator 121 to change the light amount of interference light LC.