An electric power conversion device including an inverter circuit, and a PWM signal generation unit that compares a carrier signal with a voltage command value and generates a PWM signal for driving the inverter circuit, the electric power conversion device being connected to a rotational machine or being connected to a transformer which include a winding wire to feed and receive electric power, the electric power conversion device comprising: a current sensor that detects a current fed to and received from the rotational machine or the transformer; and a diagnostic unit that diagnoses insulation deterioration of the winding wire of the rotational machine or the transformer, wherein the diagnostic unit includes a current sampling unit that obtains a current detection value by detecting a current of the current sensor at a detection timing which is a timing of a peak or a valley of the carrier signal or both the peak and the valley, an extraction unit that extracts the current detection value at the detection timing of the current as a current detection value for insulation diagnosis when the detection timing is within a certain period of time from a timing when the voltage command value and the carrier signal intersect each other, an index value calculation unit that calculates an index value used for diagnosis from the extracted current detection value, and an insulation deterioration detection unit that detects insulation deterioration of the winding wire of the rotational machine or the transformer by a change from a normal state of the calculated index value. a current sensor that detects a current fed to and received from the rotational machine or the transformer; and a diagnostic unit that diagnoses insulation deterioration of the winding wire of the rotational machine or the transformer, wherein the diagnostic unit includes a current sampling unit that obtains a current detection value by detecting a current of the current sensor at a detection timing which is a timing of a peak or a valley of the carrier signal or both the peak and the valley, an extraction unit that extracts the current detection value at the detection timing of the current as a current detection value for insulation diagnosis when the detection timing is within a certain period of time from a timing when the voltage command value and the carrier signal intersect each other, an index value calculation unit that calculates an index value used for diagnosis from the extracted current detection value, and an insulation deterioration detection unit that detects insulation deterioration of the winding wire of the rotational machine or the transformer by a change from a normal state of the calculated index value. a current sampling unit that obtains a current detection value by detecting a current of the current sensor at a detection timing which is a timing of a peak or a valley of the carrier signal or both the peak and the valley, an extraction unit that extracts the current detection value at the detection timing of the current as a current detection value for insulation diagnosis when the detection timing is within a certain period of time from a timing when the voltage command value and the carrier signal intersect each other, an index value calculation unit that calculates an index value used for diagnosis from the extracted current detection value, and an insulation deterioration detection unit that detects insulation deterioration of the winding wire of the rotational machine or the transformer by a change from a normal state of the calculated index value.