PURPOSE: To make a fine adjustment range large, the turning of optical element according to the fine adjustment extremely small and the height of the fine adjustment mechanism low, by connecting two sheets of long and short plates elastically at a right angle and connecting the end part of the short plate part elastically with respect to a base plate.

CONSTITUTION: A long plate part 31 and a short plate part 32 are connected approximately at a right angle by means of an elastic connecting part 33 which energizes the part 31 in an arrow G direction, whereby a body 34 is formed. The body 34 is fixed to a base plate 37 by means of set screws 36 in the end part 32a of the part 32 via an elastic connecting part 35 which energizes the part 32 in an arrow H direction. An optical element such as a detector 21 is fixed to the part 31a of the part 31. An adjusting screw E38 is erected on the plate 37 and when said screw is screwed down with respect to the part 32, the part 32 is turned finely around the part 35. A long hole 39 is formed in a part of the part 31 and an adjusting screw F40 is screwed through said hole to the plate 37. When the screw 40 is screwed down, the part 31 is turned finely around the part 33.