A focal length adjusting device 12 is provided with: a retention unit 70 for retaining a first lens 31; a guide unit 60 for supporting the retention unit 70 so as to allow movement along an optical axis Lx of laser light Lw; a stepper motor 80 that has a shaft body 81 and is arranged so that the central axis O1 of the shaft body 81 is orthogonal to the optical axis Lx; and a conversion unit 90 that is interposed between the shaft body 81 and the retention unit 70 and converts rotational movement of the shaft body 81 into linear movement of the retention unit 70. The conversion unit 90 comprises, between itself and the retention unit 70, a coupling unit 92 that transmits kinetic force from the conversion unit 90 to the retention unit 70 in a direction parallel to the optical axis Lx but does not transmit same in a direction parallel to the central axis O1 of the shaft body 81.