Free Electron Laser at Tokyo University of Science (FEL-SUT) was employed for exciting single vibrational modes of ammonia molecules. FEL is tuned to 10.5 μm, corresponding to the X¹A₁' (ν₂ = 1) ← (ν₂ = 0) transition of NH₃. The population of the vibrationally excited states is probed by the (2 + 1) resonance enhanced multiphoton ionization (REMPI) technique via the B ¹E" Rydberg state. Maximum excitation efficiency of ν₂ = 1 ← ν₂ = 0 transition was about 50 %, which was estimated by taking account of Franck-Condon factors and the rotational line strengths. Rotational analyses of the spectra revealed that vibrational ladder climbing up to ν₂ = 2 in NH₃ was realized for the first time. Experimental results on ν₄ vibrational excitation of NH₃ and ND₃ will be also reported.