PURPOSE: To produce an immersion oil which has good characteristics and low fluorescent property by mixing dialkylester of phthalic acid and chlorinated paraffin to produce the immersion oil.

CONSTITUTION: The immersion oil to increase the numerical aperture of an objective lens is used to increase the magnification of a microscope. The fluorescent microscope observes the fluorescence emitted by a sample when the sample is projected by various excitation beams of a light source and, therefore, the immersion oil is required to decrease the quantity of the fluorescence to be generated. The immersion oil is produced in this embodiment by mixing dialkylester of phthalic acid and chlorinated paraffin within the range of 20:80W70:30 compounding ratio by weight. This immersion oil has the good characteristics required for the immersion oil for the microscope in refractive index, Abbe number, viscosity and resolving power; furthermore the immersion oil exhibits the property of the extremely small quantity of the fluorescent to be generated by the excitation. The immersion oil having the good characteristic and the low fluorescent property is thereby obtd.