A drive system comprising: an electric motor (M); an inverter main circuit that outputs an alternating electric current for driving the electric motor (M); an electric current detector that detects an electric current value of the alternating electric current output from the inverter main circuit ; an electric current command generator that generates an electric current command value corresponding to an electric current to be supplied to the electric motor (M); a gate command generator that generates a gate command of the inverter main circuit so that the electric current command value matches the electric current value detected by the electric current detector ; an output voltage target vector calculator configured to calculate an output voltage target vector (V α ; V β ) of the inverter main circuit based on the gate command; and a rotation phase estimator configured to calculate a rotation phase estimation value (Θ est ) of the electric motor (M) based on the electric current value detected by the electric current detector and the output voltage target vector (V α ; V β ) of the inverter main circuit , in an initial estimation of when activating the inverter main circuit , wherein the electric current command value in the initial estimation is a value to supply an electric current for magnetically saturating a rotor of the electric motor (M), characterized in that the rotation phase estimator is configured to use a dynamic inductance (Ldh) of the electric motor (M) for calculating the rotation phase estimation value (Θ est ), and the dynamic inductance is a value of when the rotor of the electric motor (M) is magnetically saturated. an electric motor (M); an inverter main circuit that outputs an alternating electric current for driving the electric motor (M); an electric current detector that detects an electric current value of the alternating electric current output from the inverter main circuit ; an electric current command generator that generates an electric current command value corresponding to an electric current to be supplied to the electric motor (M); a gate command generator that generates a gate command of the inverter main circuit so that the electric current command value matches the electric current value detected by the electric current detector ; an output voltage target vector calculator configured to calculate an output voltage target vector (V α ; V β ) of the inverter main circuit based on the gate command; and a rotation phase estimator configured to calculate a rotation phase estimation value (Θ est ) of the electric motor (M) based on the electric current value detected by the electric current detector and the output voltage target vector (V α ; V β ) of the inverter main circuit , in an initial estimation of when activating the inverter main circuit , wherein the electric current command value in the initial estimation is a value to supply an electric current for magnetically saturating a rotor of the electric motor (M), characterized in that the rotation phase estimator is configured to use a dynamic inductance (Ldh) of the electric motor (M) for calculating the rotation phase estimation value (Θ est ), and the dynamic inductance is a value of when the rotor of the electric motor (M) is magnetically saturated. <EOS>