A pressure-induced structural change of a polymer isotactic poly(4-methyl-1-pentene) (P4MP1) in the melted state at 270 °C has been investigated by high-pressure in situ x-ray diffraction, where high pressures up to 1.8 kbar were applied using helium gas. The first sharp diffraction peak (FSDP) position of the melt shows a less pressure dependence than that of the normal compression using a solid pressure transmitting medium. The contraction using helium gas was about 10% at 2 kbar, smaller than about 20% at the same pressure using a solid medium. The result indicates that helium entered the interstitial space between the main chains. The helium/monomer molar ratio was estimated to be 0.3 at 2 kbar from the FSDP positions. These results suggest that the compressibility of the P4MP1 melt can be largely dependent on the pressure transmitting media. As the pore size is reversibly and continuously controllable by compression, we suggest that the P4MP1 melt can be an ideal porous liquid for investigating a novel mechanical response of the pores in a non-crystalline substance.