The self-amplified spontaneous emission (SASE) has been generated using the single bunch electron beam of 11.3MeV energy with a peak current of ~0.5kA accelerated with an L-band linac and a 1.92m long, 32 period wiggler, and the wavelength spectrum of SASE has been measured in the wide wavelength region from 50 to 200μm using a grating monochromator. We have observed the second harmonic peak at 90μm and the third harmonic peak at 60μm in the spectrum as well as the fundamental peak of SASE at 180μm. 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 2.75nJ, 73pJ, 182pJ, 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.