The relation between the luminosity and the spin-period change in X-ray binary pulsars
reflects the manner of mass accretion onto the magnetized neutron stars, and thus
provides us information on the neutron-star physical parameters related to the mass,
radius, and the surface magnetic field.  To observationally study the relation, we
analyzed X-ray light curves obtained by the MAXI all-sky survey and pulse-period data
taken by the Fermi Gamma-ray Burst Monitor pulsar project, both continuously cover 7
years since 2009 August.  In all of the 12 Be binary pulsars which exhibited significant
outbursts in these 7 years, the luminosities and the spin-frequency derivatives were
found to follow positive correlations close to the proportionality, as expected by major
disk-magnetosphere interaction models.  The coefficient of the proportionality agrees,
within a factor of 3, with the prediction by the Ghosh & Lamb (1979) when assuming a
typical mass and radius, and employing the surface magnetic field measured with the
cyclotron resonance.  I will also introduce recent results about Swift J0243.6+6124, a
new Be binary pulsar with the peak luminosity of ~2x10^39 erg s^-1, suggesting that the
source is the first Galactic ultraluminous X-ray pulsar.