  We present the result of the in-flight calibration of the effective area of
the Soft X-ray Spectrometer (SXS) onboard the Hitomi X-ray satellite using an
observation of the Crab nebula. We corrected for the artifacts when observing
high count rate sources with the X-ray microcalorimeter. We then constructed a
spectrum in the 0.5-20 keV band, which we modeled with a single power-law
continuum attenuated by an interstellar extinction. We evaluated the systematic
uncertainty upon the spectral parameters by various calibration items. In the
2-12 keV band, the SXS result is consistent with the literature values in flux
(2.20 $\pm$ 0.08) $\times$10$^{-8}$ erg s$^{-1}$ cm$^{-2}$ with a 1$\sigma$
statistical uncertainty) but is softer in the power-law index (2.19 $\pm$
0.11). The discrepancy is attributable to the systematic uncertainty of about
$+$6/$-$7% and $+$2/$-$5% respectively for the flux and the power-law index.
The softer spectrum is affected primarily by the systematic uncertainty of the
Dewar gate valve transmission and the event screening.
