  From 2005 to 2009, 25 observations of Cyg X-1 were performed with Suzaku,
achieving a total exposure of 446 ks. In all observations, the source was found
in the low/hard state, while the 1.5--12.0 keV count rate of the All-Sky
Monitor onboard RXTE varied by a factor of ~ 3. In each observation, the 10--60
keV HXD-PIN spectrum and the 60--400 keV HXD-GSO spectrum were fitted
successfully by a thermal Comptonization model plus reflection by a thick
neutral material. As the soft X-ray intensity increased, the Compton
y-parameter was found to decrease from 1.0 to 0.6, while the solid angle of
reflection to increase by ~ 30%. Also conducted was timing analysis over a
frequency range of 0.001--10 Hz. As the source became brighter in soft X-rays,
the characteristic frequency of hard X-ray variation increased from 0.03 to 0.3
Hz, while the fractional hard X-ray variation integrated over 0.001--0.01 Hz
decreased by a factor of ~ 5. The signals in the 60--200 keV band were
generally found to vary on shorter time scales than those in the 10--60 keV
band. These spectral and timing results can be consistently interpreted by
presuming that increases in the mass accretion rate cause the Comptonizing hot
corona to shrink, while the optically-thick disk to intrude deeper therein.
