To study the freezing/melting behavior of a confined CH3Cl, we performed X-ray diffraction measurements of CH3Cl confined inside the cylindrical pens of six kinds of siliceous MCM-41 with different pore radii (R = 1.2-2.9 nm) as a function of temperature. The pore-size dependence of the phase-transition temperature suddenly changes between R = 2.5 and 2.1 nm and between R = 1.8 and 1.45 nm. The CH3Cl confined to the mesopores of R greater than or equal to 1.8 nm shows a large hysteresis effect between freezing and melting and crystallizes on freezing, while the CH3Cl confined to the mesopores of R greater than or equal to 1.45 nm vitrify on cooling. Although the CH3Cl confined to the mesopores of R greater than or equal to 2.5 nm freezes into a solid with the same ferroelectric structure as the bulk, the CH3Cl confined to the mesopores of R = 1.8 and 2.1 nm crystallizes into a structure that is different from the bulk. The results are discussed on the basis of a homogeneous nucleation, as well as the effects of finite size on melting point depression and the structure of a confined solid.