Disclosed is a microscope immersion oil characterized by containing a liquid olefin polymer (A), a liquid diene polymer (B), a diaryl alkane (C) and an alkyl benzene (D). This microscope immersion oil is capable of maintaining low fluorescence of its own over a long time, while having good characteristics required for a microscope immersion oil, such as refractive index, Abbe number, viscosity and resolution. Consequently, the microscope immersion oil is particularly suitable as an immersion oil for fluorescence microscopes.