A tilt angle sensor is disclosed that is capable of detecting whether a tilt angle of a reference plane exceeds a predetermined value by means of only one threshold value. Although only one threshold value is used, the tilt angle can be measured with respect to either a clockwise or a counterclockwise direction from the untilted state. A pair of differential electrodes electrically independent of each other are formed in a shape symmetric with respect to upper and lower sections of a printed circuit board. A common electrode plate is mounted opposed to the pair of differential electrodes with a predetermined gap therebetween. The pair of differential electrodes and the common electrode is stored in a closed space formed by the printed circuit board and an oil case. A dielectric liquid is filled into the closed space in such a way that the level of the liquid varies according to the tilt of the reference plane. An output signal corresponding to the difference in capacity between two capacitors formed by associated components is produced as a tilt detection output.