A fixing device, comprising: first and second rollers (31,32) defining a fixing nip and, respectively, rotatable around their axes; a roller gear (31 G) for transmitting a torque to the first roller; a pressure adjustment mechanism for performing posture change of changing from a first posture where the first and second rollers are in pressed contact with each other at a first pressure to a second posture where the first and second rollers are in pressed contact with each other at a second pressure lower than the first pressure, and vice versa; a drive input gear device operable to receive a torque of a first rotational direction around an axis of the gear device and another torque of a second rotational direction opposite to the first direction; a ratchet mechanism for, when the drive input gear device receives the torque of the first rotational direction, transmitting the torque of the first rotational direction to the first roller and for, when the drive input gear device receives the torque of the second rotational direction, suspending a torque transmission to the first roller; and a transmission mechanism for, when the drive input gear device receives the torque of the second rotational direction, exerting a driving force to the pressure adjustment mechanism to thereby allow the posture change, characterized in that: the ratchet mechanism includes: a ratchet joint having a first ratchet tooth portion and a body portion engaged with the drive input gear device, the ratchet joint being rotatable integrally with the drive input gear device; a ratchet gear having a second ratchet tooth portion engageable with the first ratchet tooth portion (6 IT) when the drive input gear device receives the torque of the first rotational direction, and a gear part meshed with the roller gear, the ratchet gear being disposed coaxially with the ratchet joint; and a biasing member biasing the ratchet joint to thereby allow the first ratchet tooth portion to press the second ratchet tooth portion, and the drive input gear device includes a housing portion housing at least a part of the body portion of the ratchet joint and the biasing member. first and second rollers (31,32) defining a fixing nip and, respectively, rotatable around their axes; a roller gear (31 G) for transmitting a torque to the first roller; a pressure adjustment mechanism for performing posture change of changing from a first posture where the first and second rollers are in pressed contact with each other at a first pressure to a second posture where the first and second rollers are in pressed contact with each other at a second pressure lower than the first pressure, and vice versa; a drive input gear device operable to receive a torque of a first rotational direction around an axis of the gear device and another torque of a second rotational direction opposite to the first direction; a ratchet mechanism for, when the drive input gear device receives the torque of the first rotational direction, transmitting the torque of the first rotational direction to the first roller and for, when the drive input gear device receives the torque of the second rotational direction, suspending a torque transmission to the first roller; and a transmission mechanism for, when the drive input gear device receives the torque of the second rotational direction, exerting a driving force to the pressure adjustment mechanism to thereby allow the posture change, the ratchet mechanism includes: a ratchet joint having a first ratchet tooth portion and a body portion engaged with the drive input gear device, the ratchet joint being rotatable integrally with the drive input gear device; a ratchet gear having a second ratchet tooth portion engageable with the first ratchet tooth portion (6 IT) when the drive input gear device receives the torque of the first rotational direction, and a gear part meshed with the roller gear, the ratchet gear being disposed coaxially with the ratchet joint; and a biasing member biasing the ratchet joint to thereby allow the first ratchet tooth portion to press the second ratchet tooth portion, and a ratchet joint having a first ratchet tooth portion and a body portion engaged with the drive input gear device, the ratchet joint being rotatable integrally with the drive input gear device; a ratchet gear having a second ratchet tooth portion engageable with the first ratchet tooth portion (6 IT) when the drive input gear device receives the torque of the first rotational direction, and a gear part meshed with the roller gear, the ratchet gear being disposed coaxially with the ratchet joint; and a biasing member biasing the ratchet joint to thereby allow the first ratchet tooth portion to press the second ratchet tooth portion, and the drive input gear device includes a housing portion housing at least a part of the body portion of the ratchet joint and the biasing member.