The self-amplified spontaneous emission (SASE) has been generated using the single bunch electron beam of 11.3 MeV energy with a peak current of 0.5 kA accelerated with an L-band linac and a 1.92 m long, 32 period wiggler, and the wavelength spectrum of SASE has been measured in the wide wavelength region from 50 to 200 μm using a grating monochromator. We have observed the second harmonic peak at 90 μm and the third harmonic peak at 60 μm in the spectrum as well as the fundamental peak of SASE at 180 μm. Spectral widths of the fundamental, the second and the third harmonics are measured to be 2.9%, 3.6%, 5.3%, respectively. The FEL parameter is 0.032, which is estimated from the spectral width of the fundamental peak. Absolute energies per optical pulse of the fundamental, the second and the third harmonics have been derived to be 2.75 nJ, 73 pJ, 182 pJ, respectively, by calibrating the measurement system with a blackbody radiator. The second and the third harmonic peaks are concluded to be produced by nonlinear harmonic generation of SASE.