  To observationally study spin-period changes of accreting pulsars caused by
the accretion torque, the present work analyzes X-ray light curves of 12 Be
binary pulsars obtained by the MAXI/GSC all-sky survey and their pulse periods
measured by the Fermi/GBM pulsar project, both covering more than 6 years from
2009 August to 2016 March. The 12 objects were selected because they are
accompanied by clear optical identification, and accurate measurements of
surface magnetic fields. The luminosity $L$ and the spin-frequency derivatives
$\dot{\nu}$, measured during large outbursts with $L\gtrsim 1\times 10^{37}$
erg s$^{-1}$, were found to approximately follow the theoretical relations in
the accretion torque models, represented by $\dot{\nu} \propto L^{\alpha}$
($\alpha\simeq 1$), and the coefficient of proportionality between $\dot{\nu}$
and $L^{\alpha}$, agrees, within a factor of $\sim 3$, with that proposed by
Ghosh & Lamb (1979). In the course of the present study, the orbital elements
of several sources were refined.
