Isothermal phase equilibria for the methane + ethylene mixed gas hydrate system were investigated at 284 and 293 K by means of gas chromatography. In addition, the equilibrium composition in the mixed gas hydrate crystal was investigated at 293 K by means of Raman spectroscopy. At 293 K, the system exhibits behavior similar to an azeotrope as observed with vapor-liquid equilibria, with a pressure minimum point in the isothermal pressure-composition diagram. The results of Raman spectroscopic measurements reveal that the methane + ethylene mixed gas hydrate constructs the structure-I lattice in the whole composition range. This implies that solid phase transition from structure-I to -II, which is observed in the methane + ethane mixed hydrate system, for example, does not occur at 293 K in the methane + ethylene mixed hydrate system.