To provide: a mixed solvent of benzotrifluorides having high solubility with non-inflammability fluorine-based solvents, wherein the composition can be maintained very stably; a surface-treating agent that uses the mixed solvent; and the mixed solvent that is made to have non-inflammability by adjusting a composition ratio.

A mixed solvent includes: (1) bis(trifluoromethyl) benzene; and (2) C₆F₁₃OR (R is CH₃ or C₂H₅), wherein the mass ratio (1)/(2) is 1/99 to 99/1; and the total amount of both of them is at least 80 mass% based on the total of the solvent.