A beam irradiation device includes a photodetector which receives servo light; a signal processing section which generates a position detection signal based on a detection signal from the photodetector; and a control section which controls the laser light source and the actuator for beam scanning based on the position detection signal. The signal processing section performs a sampling operation with respect to the detection signal from the photodetector at two consecutive sampling timings. The signal processing section includes an A/D converting section which converts a difference between two sampling values obtained by the sampling operation into a digital signal, and a computing section which computes the position detection signal based on the digital signal from the A/D converting section.