  The Far-Infrared Surveyor (FIS) onboard the AKARI satellite has a
spectroscopic capability provided by a Fourier transform spectrometer
(FIS-FTS). FIS-FTS is the first space-borne imaging FTS dedicated to
far-infrared astronomical observations. We describe the calibration process of
the FIS-FTS and discuss its accuracy and reliability. The calibration is based
on the observational data of bright astronomical sources as well as two
instrumental sources. We have compared the FIS-FTS spectra with the spectra
obtained from the Long Wavelength Spectrometer (LWS) of the Infrared Space
Observatory (ISO) having a similar spectral coverage. The present calibration
method accurately reproduces the spectra of several solar system objects having
a reliable spectral model. Under this condition the relative uncertainty of the
calibration of the continuum is estimated to be $\pm$ 15% for SW, $\pm$ 10% for
70-85 cm^(-1) of LW, and $\pm$ 20% for 60-70 cm^(-1) of LW; and the absolute
uncertainty is estimated to be +35/-55% for SW, +35/-55% for 70-85 cm^(-1) of
LW, and +40/-60% for 60-70 cm^(-1) of LW. These values are confirmed by
comparison with theoretical models and previous observations by the ISO/LWS.
