We performed in-situ monitoring of bis-cyclopentadienyl magnesium (Cp2Mg) vapor pressure supplied into a metal-organic vapor phase epitaxy reactor. As a light source we adopted a quantum cascade laser (QCL) of 12.9 m, which is the wavelength of Cp2Mg infrared absorption. We directly configured a laser path in the reactor to construct the Cp2Mg in-situ monitoring system. We streamed Cp2Mg and measured the vapor pressure in the reactor to evaluate it and its saturation and purge times in comparison with their calculated values which were determined from the measured values in the supply line. Next, we streamed Cp2Mg and other gas species used in gallium-nitride processes one by one to investigate their saturation times with the same QCL. Characteristic features of Cp2Mg such as the largest saturation time ratio and its large fluctuation were observed. These results indicate the importance of Cp2Mg in-situ monitoring in the reactor.