To prevent deterioration in viscosity of grease, prevent the volume of the grease from decreasing over time, reduce maintenance, and reduce maintenance costs in an oilless natural gas compressor using a bearing having the grease encapsulated therein.

This natural gas compressor uses the oilless bearing having the grease encapsulated in rotary bearing parts 3, 12. The grease encapsulated in the bearing has a PFPE system as a base material expressed by F-[CF(-CF3)-CF2-O]n-CF2CF3 (n=7 to 60 in the expression), and contains PTFE as a thickener, and sodium nitrite as an additive.