A method for estimating a temperature of a coil of an electric motor, comprising the steps of: determining an initial value of each of a motor coil temperature, a temperature of a member adjacent to the motor coil and an ambient temperature of a surrounding part; determining an electric current conducted through the motor coil; computing an amount of heat generated in the motor coil from the determined electric current; and computing a temperature of the motor coil at each current time point from a computed temperature thereof at a previous time point, and a computed amount of heat generated in the motor coil and an amount of internal heat transfer from the motor coil to the adjacent member during a time interval between the previous time point and current time point, the amount of internal heat transfer from the motor coil to the adjacent member during the said time interval being computed from a difference between a temperature of the adjacent member and the temperature of the motor coil at the previous time point, the temperature of the adjacent member at the current time point being computed from the temperature of the adjacent member at the previous time point, and an amount of external heat transfer from the adjacent member to the surrounding part and the amount of heat transfer from the motor coil to the adjacent member during the said time interval, the amount of external heat transfer during the said time interval being computed from a difference between the temperature of the adjacent member and the ambient temperature of the surrounding part at the previous time point, characterized in further comprising the step of limiting an operation of the electric motor when the estimated coil temperature exceeds a prescribed threshold value, and in that the threshold value includes a first threshold value for entirely prohibiting the operation of the electric motor, a second threshold value lower than the first threshold value for prohibiting the operation of the electric motor except for a reversing movement thereof, and a third threshold value lower than the second threshold value for resuming operation of the electric motor which has been previously prohibited to operate. determining an initial value of each of a motor coil temperature, a temperature of a member adjacent to the motor coil and an ambient temperature of a surrounding part; determining an electric current conducted through the motor coil; computing an amount of heat generated in the motor coil from the determined electric current; and computing a temperature of the motor coil at each current time point from a computed temperature thereof at a previous time point, and a computed amount of heat generated in the motor coil and an amount of internal heat transfer from the motor coil to the adjacent member during a time interval between the previous time point and current time point, the amount of internal heat transfer from the motor coil to the adjacent member during the said time interval being computed from a difference between a temperature of the adjacent member and the temperature of the motor coil at the previous time point, the temperature of the adjacent member at the current time point being computed from the temperature of the adjacent member at the previous time point, and an amount of external heat transfer from the adjacent member to the surrounding part and the amount of heat transfer from the motor coil to the adjacent member during the said time interval, the amount of external heat transfer during the said time interval being computed from a difference between the temperature of the adjacent member and the ambient temperature of the surrounding part at the previous time point, characterized in further comprising the step of limiting an operation of the electric motor when the estimated coil temperature exceeds a prescribed threshold value, and in that the threshold value includes a first threshold value for entirely prohibiting the operation of the electric motor, a second threshold value lower than the first threshold value for prohibiting the operation of the electric motor except for a reversing movement thereof, and a third threshold value lower than the second threshold value for resuming operation of the electric motor which has been previously prohibited to operate.