Long-term monitoring observations of the 6.7 GHz methanol masers by Hitachi 32-m operated by Ibaraki University, which are named as “the Ibaraki 6.7 GHz Methanol Maser Monitor (iMet)”, have revealed that the periods of the flux variability of 6.7 GHz methanol masers in the five high-mass star-forming regions G05.900-0.430, G06.795-0.257, G10.472+0.027, G12.209-0.102 and G13.657-0.599 are over 1000 days. These periods are approximately twice the longest known period of 6.7 GHz methanol masers of 668 days for G196.45-1.68. The facts that the flux variation patterns show symmetric sine curves and that the luminosity of the central protostar and periods of maser flux variation are consistent with the expected period-luminosity (PL) relation suggest that the mechanism of maser flux variability of G05.900-0.430, G10.472+0.027 and G12.209-0.102 can be explained by protostellar pulsation instability. From the period-luminosity relation, central stars of these three sources are expected to be very high-mass protostars with a mass of : ∼40 M⊚ and a mass accretion rate of : ∼2×10-2 M⊚yr-1. On the other hand, G06.795-0.257 and G13.657-0.599 have luminosities that are an order of magnitude smaller than that expected from PL relation, and the variation patterns are intermittent, suggesting a variation mechanism of these sources originated from binary system.