An electric motor diagnosing device comprising: a current detection circuit for detecting current of an electric motor; a voltage detection circuit for detecting voltage of the electric motor; and a calculation processing unit which receives output of the current detection circuit and performs determination for winding short-circuit abnormality in the electric motor, wherein the calculation processing unit includes a driving state determination unit which calculates an effective value from current and voltage of the electric motor to determine whether or not the electric motor is in a driven state, a voltage imbalance rate determination unit which calculates a voltage imbalance rate from voltage of the electric motor and compares the calculated voltage imbalance rate and a set threshold to determine whether or not to perform winding short-circuit determination, characterised by an initial negative-phase-sequence current analysis unit which, when the voltage imbalance rate determination unit determines that winding short-circuit determination is to be performed, obtains an initial negative-phase-sequence current, the initial negative-phase-sequence current being an average value of negative-phase-sequence current in a normal state of the electric motor, an evaluation value analysis unit which, when the voltage imbalance rate determination unit determines that winding short-circuit determination is to be performed, calculates an evaluation value for winding short-circuit on the basis of a difference between negative-phase-sequence current calculated from the current of the electric motor detected by the current detection circuit in the driven state and the initial negative-phase-sequence current, and a winding short-circuit determination unit which performs determination for winding short-circuit of the electric motor through comparison between the evaluation value and a set threshold. a current detection circuit for detecting current of an electric motor; a voltage detection circuit for detecting voltage of the electric motor; and a calculation processing unit which receives output of the current detection circuit and performs determination for winding short-circuit abnormality in the electric motor, wherein the calculation processing unit includes a driving state determination unit which calculates an effective value from current and voltage of the electric motor to determine whether or not the electric motor is in a driven state, a voltage imbalance rate determination unit which calculates a voltage imbalance rate from voltage of the electric motor and compares the calculated voltage imbalance rate and a set threshold to determine whether or not to perform winding short-circuit determination, characterised by an initial negative-phase-sequence current analysis unit which, when the voltage imbalance rate determination unit determines that winding short-circuit determination is to be performed, obtains an initial negative-phase-sequence current, the initial negative-phase-sequence current being an average value of negative-phase-sequence current in a normal state of the electric motor, an evaluation value analysis unit which, when the voltage imbalance rate determination unit determines that winding short-circuit determination is to be performed, calculates an evaluation value for winding short-circuit on the basis of a difference between negative-phase-sequence current calculated from the current of the electric motor detected by the current detection circuit in the driven state and the initial negative-phase-sequence current, and a winding short-circuit determination unit which performs determination for winding short-circuit of the electric motor through comparison between the evaluation value and a set threshold. a driving state determination unit which calculates an effective value from current and voltage of the electric motor to determine whether or not the electric motor is in a driven state, a voltage imbalance rate determination unit which calculates a voltage imbalance rate from voltage of the electric motor and compares the calculated voltage imbalance rate and a set threshold to determine whether or not to perform winding short-circuit determination, characterised by an initial negative-phase-sequence current analysis unit which, when the voltage imbalance rate determination unit determines that winding short-circuit determination is to be performed, obtains an initial negative-phase-sequence current, the initial negative-phase-sequence current being an average value of negative-phase-sequence current in a normal state of the electric motor, an evaluation value analysis unit which, when the voltage imbalance rate determination unit determines that winding short-circuit determination is to be performed, calculates an evaluation value for winding short-circuit on the basis of a difference between negative-phase-sequence current calculated from the current of the electric motor detected by the current detection circuit in the driven state and the initial negative-phase-sequence current, and a winding short-circuit determination unit which performs determination for winding short-circuit of the electric motor through comparison between the evaluation value and a set threshold.