A 1-(m in diameter polystyrene bead coated with lectin is trapped by optical tweezers which is formed by a focused laser beam (740 nm). A leucocyte adhered to the bottom of sample cell is chosen to close to the trapped bead. When their stable combination is confirmed, the leucocyte is displaced by moving the sample stage. A "tether" is then formed between the trapped bead and the membrane. The force acting on the tether is measured by a detector equipped on the optical tweezers system. The deformation of the membrane is diverse in different contact conditions. When the contact area is small, the tether is very thin and the force on it is 7.4 pN (10-12 N), while the tether is much thicker under a larger contact area and its corresponding force is about 14 pN. It is presumed that the latter is related to the deformation of cytoskeleton.