OBJECTIVE

PROBLEM TO BE SOLVED: To provide an objective lens which functions to correct defocusing due to temperature variation and eliminates the need of focusing operation. SOLUTION: The objective lens which has lenses 10, a lens holding frame 20 for holding the lenses 10, an intermediate barrel 30 containing the lens holding frame 20, a housing 40 storing the intermediate cylinder 30, and a nut 50 threadably engaging the outer periphery of the housing 40 is characterized by that the nut 50 is made of cast iron for low expansion and the length ratio of the nut 50 and housing 40 along the optical axis is so set as to make constant the quantity of defocusing of the lens 10 caused by temperature variation.