A fault monitoring method for a robot (10) or other work machine (30) is disclosed, in which the period that starts when the robot or the work machine starts moving and ends when the work machine stops moving is set as one unit, the difference between the maximum torque and the minimum torque within the unit is set as a maximum fluctuation range, a fluctuation range control value is set on the basis of the maximum fluctuation range, an average is taken of the maximum fluctuation ranges at plural points during operation, a second average value (t1, t10, t100) is determined, and a fault is determined to have occurred when the second average value exceeds the fluctuation range control value.