There is provided a detector apparatus capable of detecting the position or tilt angle of a sample stage with high resolution and high reliability. The detector apparatus (100) is operative to detect the position or tilt angle of the sample stage (2), and has a potentiometer (10) for detecting the position or tilt angle of the sample stage (2), an encoder (20) for detecting the position or tilt angle of the sample stage (2), and a computing unit (30) for calculating the position or tilt angle of the sample stage (2), based both on an output signal from the potentiometer (10) and on an output signal from the encoder (20).