To clean a relatively heavy liquid from small amounts of relatively light liquid the liquids are first introduced into a container, in which the light liquid is allowed to collect on the surface of the heavy liquid. Twisting is imparted over a full length of a light diffusing yarn continuously at a frequency of more than one time with a length five times as long as the width of the flat yarn. The second inductor is electrically connected to the first inductor in series at one end. ,provided that when X is \u2014CO\u2014, then B is N,or a pharmaceutically acceptable salt thereof. Also, a wrap-around phenomenon of the developer can be prevented.