An artificial knee joint comprising: a thigh joint part provided on a side of a thigh part; a lower leg part coupled to the thigh joint part and provided to be rotatable around a knee shaft; a thigh part inclination angle acquisition unit that acquires an inclination angle formed by the thigh part relative to a vertical line passing through the knee shaft; a rotational resistance control unit that weakens a rotational resistance of the knee shaft in accordance with a transition from a positive inclination angle formed when the thigh part is inclined rearward relative to the vertical line to a negative inclination angle formed when the thigh part is inclined forward relative to the vertical line, characterized by further comprising: a thigh part inclination angular speed acquisition unit that acquires an angular speed of the inclination angle of the thigh part; and a delay time acquisition unit that acquires a delay time that elapses until a result of control by the rotational resistance control unit is reflected in the rotational resistance, wherein when the angular speed of the thigh part is negative, the rotational resistance control unit starts controlling the rotational resistance of the knee shaft to be weaker while the inclination angle of the thigh part is larger than a predetermined threshold value so as to prevent the inclination angle of the thigh part from falling below the threshold value during the delay time. a thigh joint part provided on a side of a thigh part; a lower leg part coupled to the thigh joint part and provided to be rotatable around a knee shaft; a thigh part inclination angle acquisition unit that acquires an inclination angle formed by the thigh part relative to a vertical line passing through the knee shaft; a rotational resistance control unit that weakens a rotational resistance of the knee shaft in accordance with a transition from a positive inclination angle formed when the thigh part is inclined rearward relative to the vertical line to a negative inclination angle formed when the thigh part is inclined forward relative to the vertical line, characterized by further comprising: a thigh part inclination angular speed acquisition unit that acquires an angular speed of the inclination angle of the thigh part; and a delay time acquisition unit that acquires a delay time that elapses until a result of control by the rotational resistance control unit is reflected in the rotational resistance, wherein when the angular speed of the thigh part is negative, the rotational resistance control unit starts controlling the rotational resistance of the knee shaft to be weaker while the inclination angle of the thigh part is larger than a predetermined threshold value so as to prevent the inclination angle of the thigh part from falling below the threshold value during the delay time. a thigh part inclination angular speed acquisition unit that acquires an angular speed of the inclination angle of the thigh part; and a delay time acquisition unit that acquires a delay time that elapses until a result of control by the rotational resistance control unit is reflected in the rotational resistance, wherein when the angular speed of the thigh part is negative, the rotational resistance control unit starts controlling the rotational resistance of the knee shaft to be weaker while the inclination angle of the thigh part is larger than a predetermined threshold value so as to prevent the inclination angle of the thigh part from falling below the threshold value during the delay time.