In a beam irradiation device, laser light emitted from a laser light source is entered into a mirror. An actuator pivotally moves the mirror into which laser light is entered, whereby a targeted area is scanned with the laser light. Servo light emitted from a semiconductor laser is entered into a hologram element. The hologram element is pivotally moved with the pivotal movement of the mirror. A diffraction pattern is formed on an exit surface of the hologram element. A photodetector receives servo light transmitted through the hologram element to output a signal depending on a light receiving position of the servo light. The scan width of servo light on the photodetector is increased by a diffraction function of the hologram element.