To study the freezing/melting behavior of a confined CH3OH, we performed x-ray diffraction measurements of CH3OH confined inside the cylindrical pores of seven kinds of regular mesoporous adsorbents (MCM-41 and SBA-15) with different pore radii (r=1.2, 2.1, 2.5, 3.9, 4.5, 5.3, and 7.0 nm) as a function of temperature. The freezing/melting behavior depends markedly upon the pore size. Within the pores of r less than or equal to 3.9 nm, the confined CH3OH vitrifies on freezing. On the other hand, cooling of the CH3OH confined to the pores of r greater than or equal to 4.5 nm results in crystallization of the liquid. Within the pores of r=5.3 nm, the crystallization proceeds in two steps: prefreezing first occurs and then it transforms into a crystalline solid with the same structure as that of the bulk alpha phase. The prefreezing temperature seems to lower steeply with decreasing pore-size and to approach the freezing temperature for the pores of r=4.5 nm. Cooling of the CH3OH confined to the pores of r=7.0 nm results in formation of a crystalline solid with the same structure as that of the bulk beta phase and it does not transform into the low temperature alpha phase on further cooling down to 30 K, leading to the appearance of a glassy crystal with the beta phase structure. A large hysteresis effect between freezing and melting is observed. A mechanism of the vitrification is discussed.