An abnormality detection method for detecting an abnormality of an encoder for sensing a rotational position of a motor that drives an output shaft of an operation system, using a controller configured to output a speed command designating the rotational position of the motor, and commanded position information indicating the rotational position of the motor according to the speed command, a driver configured to receive the speed command output from the controller and an output signal output from the encoder, and to control drive of the motor based on the speed command and the output signal, and an abnormality detection device configured to detect the abnormality of the encoder, a damping filter for removing a vibration component of the operation system from the commanded position information; a delay control filter for compensating the commanded position information for a time delay caused by a delay in drive control of the motor; and the damping filter including a set of a plurality of damping filters with filter constants different from each other; the delay control filter including a set of a plurality of delay control filters with time constants different from each other; the method comprising: a step of using the abnormality detection device, a step of obtaining corrected position information from the controller and the output signal from the encoder, the corrected position information being obtained by removing a vibration component of the operation system and compensating for a time delay caused by a delay in drive control of the motor; and a step of determining, after comparing the corrected position information with detected position information on the motor calculated based on the output signal, the abnormality of the encoder, if there is a difference greater than or equal to a predetermined value between the corrected position information and the detected position information, and the method further comprising, a step of using the controller, a step of selecting one of the plurality of damping filters which has one of the filter constants corresponding to a weight of an attachment load held by the operation system, a step of selecting one of the plurality of delay control filters which has one of the time constants corresponding to a filter constant of the one of the damping filters selected by an applicable damping filter determiner, wherein the controller removes, from the commanded position information, the vibration component corresponding to a weight of an attachment load held by the operation system, and compensates for the time delay to obtain the corrected position information by applying the selected damping filter and the selected delay control filter to the commanded position information. a controller configured to output a speed command designating the rotational position of the motor, and commanded position information indicating the rotational position of the motor according to the speed command, a driver configured to receive the speed command output from the controller and an output signal output from the encoder, and to control drive of the motor based on the speed command and the output signal, and an abnormality detection device configured to detect the abnormality of the encoder, a damping filter for removing a vibration component of the operation system from the commanded position information; a delay control filter for compensating the commanded position information for a time delay caused by a delay in drive control of the motor; and the damping filter including a set of a plurality of damping filters with filter constants different from each other; the delay control filter including a set of a plurality of delay control filters with time constants different from each other; a step of using the abnormality detection device, a step of obtaining corrected position information from the controller and the output signal from the encoder, the corrected position information being obtained by removing a vibration component of the operation system and compensating for a time delay caused by a delay in drive control of the motor; and a step of determining, after comparing the corrected position information with detected position information on the motor calculated based on the output signal, the abnormality of the encoder, if there is a difference greater than or equal to a predetermined value between the corrected position information and the detected position information, and the method further comprising, a step of using the controller, a step of selecting one of the plurality of damping filters which has one of the filter constants corresponding to a weight of an attachment load held by the operation system, a step of selecting one of the plurality of delay control filters which has one of the time constants corresponding to a filter constant of the one of the damping filters selected by an applicable damping filter determiner, wherein the controller removes, from the commanded position information, the vibration component corresponding to a weight of an attachment load held by the operation system, and compensates for the time delay to obtain the corrected position information by applying the selected damping filter and the selected delay control filter to the commanded position information.