A fixing device (14) includes a fixing roller (31) which is in contact with an unfixed image formed on a printing medium (91) with toner (93) and a pressure roller (32) which is in contact with the fixing roller (31). The fixing device (14) causes the fixing roller (31) and the pressure roller (32) to sandwich and feed the printing medium (91), so as to fix the unfixed image onto the printing medium (91). The fixing device (14) gives a holding electric field which is an electric field in a direction for holding a reverse polarity toner (92) on the printing medium (91). The reverse polarity toner (92) has a polarity opposite to that of the toner (93) which forms an image on the printing medium. Thus, it is possible to avoid a troulile caused by the adhesion of the reverse polarity toner (92) to the pressure roller (32). Therefore, it is possible to avoid an image failure caused by the reverse polarity toner, maintain a normal image forming operation, and secure a satisfactory image quality and the life of each means even in long-term use.