[en] We have investigated the possibility of tunable Infrared Free-Electron Laser with Test Linac of Pohang Accelerator Laboratory through one dimensional simulation which includes energy spread and space charge effects and 3-D particle simulation that ingnores space charge force but takes into account the energy spread and the emittance of the electron beam and the diffraction of the electromagnetic wave. The enhanced current density of the Test Linac makes it feasible to amplify the 4.2 kW signal of 10.6 μm radiation to 200 MW level. Extending the design parameters, electron beam energy (20-60 MeV), wiggler field strength (around 3 kG), and radiation wavelength (10-90 μm), we have revealed the requisites for the design of the tunable radiation source and the expected gains in that frequency range. It is shown to generate more than 100 MW in the tunable range. (author)