A rotor shaft sealing method for an oil-free rotary compressor is provided, with which occurrence of lubrication oil intrusion into the compression chamber of the compressor which is liable to occur when negative pressure is produced in the compression chamber, is prevented. With a rotor shaft sealing structure composed such that two shaft seal means are provided in the rotor casing between the oil lubricated bearing and the compression chamber such that an annular airspace is formed between the two shaft seal means, at least one communicating hole is provided to communicate the annular airspace to the outside of the rotor casing, and the annular airspace of the male rotor shaft sealing part and the annular airspace of the female rotor shaft sealing part are connected by a between-rotor shaft communication passage, pressurized air is supplied to the annular airspaces by which lubrication oil intrusion into the compression chamber is prevented.