A liquid sample to be analyzed is subjected to a vaporizing step. The vaporized components are submitted to a first separation stage by introducing said components into a packed chromatography column and flowing carrier gas through said column. The effluent of the first stage is passed through a flow dividing chamber. A second separation stage is performed by passing the effluent of the flow dividing chamber to a capillary chromatography column and flowing carrier gas therethrough. After a predetermined period of time, flushing gas is introduced through the flow dividing chamber and flowed through the first-stage packed column in a direction opposite to the original direction of flow while the direction of flow remains unchanged in the capillary column.