A laser device applied to a light source of laser scanning units (LSU) including a laser diode disposed on an accommodation hole of a flange. A collimator lens is arranged on one end of an inner tube part of a holder while the other end of the inner tube part faces the laser diode. The laser beam passes through the inner tube part of the holder and the collimator lens, then projects out. A lug extends radially from outer surface of the holder and a plurality of axial guide slots are disposed on the lug. A plurality of guide pins corresponding to the guide slots are arranged on the flange. An adhesive is located between the guide pins and the guide slots and spaced apart from the collimator lens.