A removable fastener insulating device comprising a heat resistant cover configured for insulating a fastener. Step : a step in which an aprotic organic solvent is used in an amount at least 5 times by weight the amount of the polyoxyalkylene derivative to dissolve the polyoxyalkylene derivative therein and give a solution; step : a step in which an adsorbent comprising an oxide containing at least one of aluminum and silicon is added to the solution in an amount 0. According to the insulation transformer, the operation deficiency of the insulation transformer can be reduced and thus the signal transmission efficiency can be improved. Furthermore, the filtering is such that, in the case that the pair of audio input signals includes a panned signal component, the listener listening to the pair of output signals through headphones is provided with the sensation that the panned signal component emanates from a virtual sound source at a center location between the virtual speaker locations.