We report for the first time to our knowledge the experimental realization of a storage ring free-electron laser (FEL) in the middle-infrared (MIR) region. A technique to adjust the optical cavity using higher harmonic FELs was developed for a fundamental FEL in the MIR region. The MIR FELs were oscillated in the wavelength region of 2475 to $2673\mathrm{nm}$ , and the relative linewidth was $5\times {10}^{-4}$ . A quasi-monochromatic x-ray beam with an energy of $700\mathrm{keV}$ was generated using FEL Compton backscattering. We were able to realize a quasi-monochromatic x-ray beam, whose energy is difficult to generate even in advanced synchrotron radiation facilities.