A fixing apparatus, an image forming apparatus, and a nip width controlling method, capable of preventing deterioration in fixing quality by controlling the nip width with high accuracy, are provided. The fixing apparatus includes a pair of fixing members that are in pressure-contact with each other and in which at least one of the members rotates to thereby form a fixing nip that sandwiches and conveys a recording medium; a moving section that moves the pair of fixing members relative to each other in a direction in which the pair of fixing members are brought into pressure-contact with each other or in a direction opposite thereto; a detector that detects rotational torque for rotating any one of the pair of fixing members; and a control section that controls the moving section such that the pair of fixing members are moved relative to each other corresponding to the detected rotational torque.