A filter unit includes: a feeding shaft 24 around which an unused part of a roll filter 18 is wound; a take-up shaft 25 for taking up the filter from the feeding shaft; a motor 40 for rotating and driving the take-up shaft so as to take up the filter; a filter unit housing 26 for housing the feeding shaft and the take-up shaft and including, between the both shafts, an opening for exposing the filter; rotation detection patterns 42 that rotate in connection with transfer of the filter caused by taking up the filter and are placed at predetermined intervals in a circumferential direction in which the rotation detection patterns rotate; a pattern sensor 43 for detecting rotation of the rotation detection patterns and outputting a detection signal whose amplitude fluctuates in accordance with the predetermined intervals; and a used filter amount detecting unit for detecting a used amount of the filter based on a correspondence between a period at which the amplitude of the detection signal fluctuates and an amount of the filter taken up by the take-up shaft. Since the period at which the amplitude of the detection signal fluctuates corresponds to the used amount of the filter, the used amount of the filter can be detected precisely.