A closed specimen collection system for blood samples includes a collection tube that has a shaped nozzle adapted to fit sealably into a port connected to a centrally inserted venal catheter. The ground electrode includes: a thick portion provided on a base end side, a thin portion provided on a distal end side, and a stepped portion provided on an inner peripheral surface between the thick portion and the thin portion. Next, the mask layer is etched to expose portions of the polysilicon layer, and to define a first patterned region, a second patterned region and a third patterned region located between the first and second patterned regions.