Oscillator-type infrared Free Electron Lasers (FELs) have been used for various applications in material science, chemistry, biology, and so on. Extraction efficiency is the energy conversion efficiency from the kinetic energy of electron beam to the electromagnetic energy of laser light and a key parameter, which determines performances of an FEL oscillator. High extraction efficiency should be achieved to generate intense and ultra-short FEL pulses. In this article, dependence of the extraction efficiency of FEL oscillators on operation parameters and recent activity for increasing the extraction efficiency in Kyoto University FEL are discussed.