An electrical energy storage unit (120) of an excavator includes an electrical energy storage device (19) that accumulates electric power generated by a generator and an electric circuit that controls a charge/discharge operation of the electrical energy storage device. An abnormality detection part detects an abnormality of the electrical energy storage unit (120). An electrical energy storage state estimation part of the abnormality detection part obtains an estimation value of an electrical energy storage state of the electrical energy storage device (19) based on a voltage detection value obtained by a voltage detection part and a current detection value obtained by a current detection part. An abnormality determination part of the abnormality detection part makes an abnormality determination based on the estimation value obtained by the electrical energy storage state estimation part.