PURPOSE: To prevent a mounting part from being shaky while securing easiness in maintenance by fixing a conical mirror fixing means by the use of a wedging means for attaching a wedge in a direction perpendicular to the attaching/ detaching direction of a conical mirror on a base plate.

CONSTITUTION: The base plate 3 is screwed and fixed on a fixing side member 25 by screwing parts 25a and 25b. An insertion hole in which a wedge plate 4 is inserted from a side surface direction is formed on the base plate 3, and a sub-reflection mirror 1 is fixed on an attaching part 5. Next, when the conical mirror 2 is inserted from the center part of the sub-reflection mirror 1 and its shaft part 20 reaches the insertion hole, the wedge plate 4 is inserted and the shaft part 20 is engaged and fixed in an inverted U-shaped mortise. Then, the wedge plate 4 is moved and fixed to the base plate 3 by a fixing screw 7 at a position where the conical mirror 2 comes in contact with the sub- reflection mirror 1. Therefore, the conical mirror 2 is fixed without being screwed, so that maintenance is facilitated.