To provide a polyacetal resin composition which is highly rigid and has excellent surface hardness, sliding characteristics, etc. The lens fastener pushes the circumferential part of the optical lens, thereby to deform the elastic member to adjust the position of a focal point of the optical lens. A non-uniform two-dimensional electron gas conduction channel is formed in the carrier supplying layer, the two-dimensional electron gas conduction channel having a non-uniform profile between the gate and drain elements. The printhead has a plurality of nozzle chambers each corresponding to a respective nozzle, with a plurality of the heater elements being disposed within each chamber. Connection to the pull electrode is sufficient to operate the device as a simple switch, while connection to a contact electrode is useful to operate the device in a manner analogous to a relay.