A separation device for separating a filtrate from a sample fluid, especially for extracting plasma from whole blood, comprising a sample container for receiving the sample fluid and a filter plunger to be introduced under seal into the sample container, which filter plunger has a filter element at its front end and a grip element on the opposite end and will receive in its interior the filtrate obtained. After insertion of the filter plunger into the sample container an annular chamber is formed between the inner wall of the sample container and the outer wall of the filter plunger, which is sealed against the exterior by a sealing lip and in which an air cushion is formed upon introduction of the filter plunger into the sample container, which acts on the sample fluid. A flow connection is provided between the annular chamber and the front side of the filter element after insertion of the filter plunger is terminated.