The present invention provides a process for continuously producing a polymer sheet having an excellent surface smoothness, an apparatus for producing such a polymer sheet, and an optical polymer sheet produced by such a process. By forming extraction electrodes for controlling emitted electrons around an anode on a front substrate, the triode field emission display using carbon nanotubes has a simple structure like a diode FED, thereby facilitating manufacture using vapor deposition of carbon nanotubes and allowing control of anode current using extraction electrodes. The elongated body also comprises a second coupling member at the opposite end from the elongated body from the first coupling member for coupling the second daughter board in a substantially perpendicular orientation to the longitudinal direction on a second side of the mother board. A sensing substance such as enzyme may be placed at the electrode section to detect a current value corresponding to the concentration or quantity of an object under test placed between a counter electrode and the biodevice. The steering mechanism includes a power conversion mechanism arranged to convert power of the electric motor into turning of the steering shaft around the steering axis.