An MIR-FEL facility, KU-FEL, has been constructed for application to energy science, making use of wavelength tunability of FEL enabling to enhance the energy conversion efficiency of the biochemical reactions. The FEL system consists of a compact S-band linac and an undulator to generate 4-13 μm FEL. The linac consists of a thermionic rf gun and a 3-m accelerator tube fed by 10 MW and 20 MW rf powers, respectively. Recent progresses such as an rf input pulse-shape control technique against the back-bombardment effect inherent in thermionic rf gun, and a newly developed beam diagnostic method employing a computer tomography technique have enabled successful production of 40 MeV, 40 mA and 4 μsec electron beams so far, leading to the first observation of 9.2 μm spontaneous emission. Further optimization parameters of both the electron beam and the optical cavity are being pursued for an FEL losing in the near future. Also studied are a new concept of superconducting micro-undulator and a triode themionic rf gun aiming at a drastic reduction of the backbombardment for extending the compact IR-FEL performance and its application.