PURPOSE: To prevent the external vibration from affecting an objective lens by providing a support shaft supporting rotably a support arm between one end of a holding arm holding an objective lens and the other end of the support arm holding a counter weight.

CONSTITUTION: A bearing 10 holds a sleeve 17 freely slidably in the optical axis direction shown in the arrow E. The objective lens 2 is supported by the sleeve 17. Further, the sleeve 17 is provided with pins 14a, 14b. On the other hand, a long hole 19 is formed respectively to one end of the support arms 13a, 13b and the pins 14a, 14b are inserted respectively to the long holes 19 to be moved in the lengthwise direction of the long holes 19. On the other hand, the support shaft 15 is provided to a fixed stand 18 and the support arms 13a, 13b are fitted to the support shaft 15 rotatably in the direction of the arrow F. The position where the support arms 13a, 13b are fitted to the support shaft 15 is a position where the rotary balance between the objective lens 2 supported by the sleeve 17 and the counter weight is kept around the support shaft 15.