40 is installed at one sidewall of the trench 5. The micromirror 30 is driven by an electrostatic force induced by interaction between the lower and side electrodes 37 and 40 and the driving unit 30a. In other words, if an attractive electrostatic force acts between the lower electrode 37 and the driving unit 30a, the micromirror 30 rotates clockwise about the torsion bar 25. As the micromirror 30 rotates, an attractive electrostatic force between the driving unit 30a and the side electrode 40 increases in strength causing the micromirror 30 to continue rotating until it stands vertically. After the electrostatic driving force is removed, the micromirror 30 is restored to a horizontal state due to the elastic restoring force of the torsion bar 25.
In this case, anelectrode surface, upon which an electrostatic force acts, is formed on the whole micromirror 30. Thus, if voltage is applied to the micromirror 30, an electrostatic force acts between the driving unit 30a and the lower and side electrodes 37 and 40 and between the reflecting unit 30b and the lower and side electrodes 37 and 40. The distance from the reflecting unit 30b to the lower and side electrodes 37 and 40 is greater than the distance from the driving unit 30a to the lower and side electrodes 37 and 40; however, the substrate 15 is formed of silicon and the dielectric constant of silicon is at least 10 times greater than the dielectric constant of air. Thus, an electrostatic force can be strongly exerted between the