Optical fibers (1, 2) are disposed so as to oppose each other with the axes thereof deviating from each other so that an attenuation becomes greater than a designated attenuation, and the opposed portion of the optical fibers is heated and fused by heating means (3) such as an arc discharge heating portion including electrodes (3a,3b) and a power supply portion (3c). The resulting heating fusion portion is repeatedly reheated under the control of a control portion (4) until the designated attenuation is attained, by utilizing the characteristics that the attenuation drops when the heating fusion portion is reheated. In this way, fine adjustment of the attenuation can be carried out.
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