One end of an optical fiber for oscillation is held in a holding groove having a V-shaped section, which holding groove is formed on the upper face of a lower block. On an upper block, a slit is formed in the place that is counter to the holding groove of the lower block. A sapphire rod can be inserted from above into the slit, and is guided vertically in a slidable manner in the slit. The sapphire rod is brought in contact with the upper face of the optical fiber for oscillation, and is supported to be slidable over both side inner wall surfaces at the lower end of the slit. An extent of screwing of bolt is changed to adjust a spring pressure of a compressive coil spring, that is, a press force against the optical fiber for oscillation.