  We analyzed continuous MAXI/GSC data of the X-ray binary pulsar 4U 1626-67
from 2009 October to 2013 September, and determined the pulse period and the
pulse-period derivative for every 60-d interval by the epoch folding method.
The obtained periods are consistent with those provided by the Fermi/GBM pulsar
project. In all the 60-d intervals, the pulsar was observed to spin up, with
the spin-up rate positively correlated with the 2-20 keV flux. We applied the
accretion torque model proposed by Ghosh & Lamb (1979, ApJ, 234, 296) to the
MAXI/GSC data, as well as the past data including both spin-up and spin-down
phases. The Ghosh & Lamb relation was confirmed to successfully explain the
observed relation between the spin-up/down rate and the flux. By comparing the
model-predicted luminosity with the observed flux, the source distance was
constrained as 5-13 kpc, which is consistent with that by Chakrabarty (1998,
ApJ, 492, 342). Conversely, if the source distance is assumed, the data can
constrain the mass and radius of the neutron star, because the Ghosh & Lamb
model depends on these parameters. We attempted this idea, and found that an
assumed distance of, e.g., 10 kpc gives a mass in the range of 1.81-1.90 solar
mass, and a radius of 11.4-11.5 km, although these results are still subject to
considerable systematic uncertainties other than that in the distance.
