A device reduces dust for safely preventing laser-irradiation capacity from decreasing due to contamination and reliably reducing iron loss of a grain oriented electrical steel sheet. The device improves iron loss properties of a grain oriented electrical steel sheet by irradiating its surface with laser to reduce iron loss, wherein, distance between a laser beam emission port and a laser irradiation point is L (mm); laser irradiation angle formed by a line linking the emission port and the irradiation point with respect to a direction vertical to the sheet is θ (°); and L≧50, the emission port is positioned such that L and θ satisfy: 60−0.3L≦θ≦60 when L≦100; 40−0.1L≦θ≦60 when 100 400.