A beam irradiation device includes: a first light source for emitting laser light; an actuator for moving a scanning section for receiving the laser light to scan a target area with the laser light; a second light source movable with the scanning section and adapted for emitting diffused light; a light receiving position detecting device for receiving the diffused light to output a signal depending on a position of receiving the diffused light; and a light projecting element, disposed at a position closer to the light receiving position detecting device with respect to an intermediate position between the second light source and the light receiving position detecting device, for projecting an emission position to be defined by the second light source on the light receiving position detecting device via a predetermined projection area.