A lubricating mechanism for a piston type compressor in a refrigeration system. A cam plate is mounted on a drive shaft for integral rotation therewith in a crank chamber, which is defined in a casing. Pistons are coupled to the cam plate and reciprocate in cylinder bores extending parallel to the drive shaft. Each piston compresses refrigerant gas containing lubricating oil mist and discharges the compressed refrigerant gas from the compressor during rotation of the cam plate. The refrigerant gas is supplied into the crank chamber and is circulated in the casing. The lubricating oil is supplied to various moving parts from a location near the drive shaft. An oil pan is provided outside and to the side of the casing for collecting lubricating oil. A recovering passage connects the oil pan with the crank chamber to convey the lubricating oil from the crank chamber to the oil pan for collection. A guide passage guides the lubricating oil collected in the oil pan to the location near the drive shaft using gravitational force. By mounting the oil pan on the side of the casing, the oil pan collects relatively less liquefied refrigerant and more oil, and thus improves lubrication of the compressor.