To compensate defocusing along an optical axis due to the thermal expansion of a microscope main body not through a complicated electronic control system.

This device has an objective 2 and a stage 3 which is constituted movably for focus adjustment. The stage 3 is constituted movably along the optical axis 22 of the objective 2 through a rack 4 and a driving gear 5. When the microscope 1 is heated by an electronic structure member 6 and/or lighting device 7 which is built in, the microscope 1 is easy to expand, and consequently the microscope 1 becomes out of focus. For the compensation of the defocusing, two rods 8 and 9 which are coupled with each other and have different thermal expansion are so interposed that the rods thermally expand in mutually opposite directions.