We present a novel blood filter unit that is designed to separate blood plasma from whole blood by simple magnetic actuation. A non-diluted blood sample is dropped into the filter unit and magnetic attraction is applied to squeeze out only blood plasma while blood particles are filtered by membranes stacked in the filter unit. The new filter device yields good filtering performance with nearly perfect filtering efficiency (∼99.999%), high plasma recovery (∼30%), low blood consumption (<50 μl), and fast operation (∼1 min). Because it is simple to operate and is attachable to any kind of pre-made biochip, it has commercial potential in various lab-on-a-chip applications for blood tests.