Conventionally, there is an image forming apparatus such as a Multi-functional Peripheral (hereinafter referred to as an “MFP”), a printer and the like.  The image forming apparatus comprises a fixing member, a press roller and a switching mechanism.  For example, it is assumed that the fixing member is a cylindrical member such as a heat roller or a fixing belt.  The switching mechanism switches a position of the press roller.  The switching mechanism is provided with a cam for switching the position of the press roller.  For example, at the time of passing an image receiving medium (hereinafter referred to as “sheet-passing time”), the switching mechanism makes the press roller contact with the fixing member.  Hereinafter, a position where the press roller is contacted with the fixing member is referred to as a “contact position”.  On the other hand, at the time of not passing the image receiving medium (hereinafter referred to as a “non-sheet-passing time”), the switching mechanism separates the press roller from the fixing member.  Hereinafter, a position where the press roller is separated from the fixing member is referred to as a “separation position”.  There is a possibility that a motion sound generates due to a load fluctuation at the time of switching from the contact position to the separation position (hereinafter referred to as a “switch time”). 
Especially, the pressing force of the press roller is set to be large in a color copier of which the sheet-passing speed is high (hereinafter referred to as a “high-speed color copier”) to guarantee the fixing property.  The press roller presses the fixing member to form a nip.  To guarantee the fixing property, it is necessary to guarantee that the nip is wide.  To guarantee that the nip is wide, it is considered to increase an amount of engagement of the press roller with the fixing member (hereinafter referred to as an “amount of engagement”).  Further, reducing the heat capacity of the fixing member is effective to shorten a warming-up time and a first print time.  It is required to separate the press roller from the fixing member absolutely at a non-sheet-passing time to reduce the heat capacity of the fixing member.  However, if the amount of engagement is increased at the sheet-passing time, the movement distance of the press roller from the contact position to the separation position becomes long.  At this time, to realize miniaturization and low cost, enough space for the switching mechanism can hardly be guaranteed in the high-speed color copier.  Thus, the load fluctuation at the switch time tends to become large in the high-speed color copier, and therefore there is a possibility that the motion sound becomes louder.