  Results of the long-term monitoring observations of the 6.7 GHz Class II
methanol masers associated with the four high-mass star-forming regions by
Hitachi 32-m radio telescope are presented. We detected periodic flux
variability in G06.795-0.257, G10.472+0.027, G12.209-0.102, and G13.657-0.599
with the periods of 968, 1624, 1272, and 1266 d, respectively, although the
detected period is tentative due to the short monitoring term relative to the
estimated period. The facts that the flux variation patterns show the symmetric
sine curves and that the luminosities of the central protostar and periods of
maser flux variation are consistent with the expected period-luminosity (PL)
relation suggest that the mechanisms of maser flux variability of G10.472+0.027
and G12.209-0.102 can be explained by protostellar pulsation instability. From
the PL relation, central stars of these two sources are expected to be very
high-mass protostars with a mass of 40 M and to have the mass accretion rate of
2*10-2 M yr-1. On the other hand, G06.795-0.257 and G13.657-0.599 have the
intermittent variation patterns and have luminosities that are an order of
magnitude smaller than those expected from PL relation, suggesting variation
mechanisms of these sources originated from binary system. Since almost all the
maser features vary with the same period regardless of its geometry, periodic
accretion model may be appropriate mechanisms for flux variability in
G06.795-0.257 and G13.657-0.599.
