A drive system comprising: an electric motor; an inverter main circuit that outputs an alternating electric current for driving the electric motor; 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; 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 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, characterized in that the rotation phase estimator is configured to use a dynamic inductance (Ldh) of the electric motor for calculating the rotation phase estimation value (Θ est), and the dynamic inductance is a value of when the rotor of the electric motor is magnetically saturated. an electric motor; an inverter main circuit that outputs an alternating electric current for driving the electric motor; 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; 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 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, characterized in that the rotation phase estimator is configured to use a dynamic inductance (Ldh) of the electric motor for calculating the rotation phase estimation value (Θ est), and the dynamic inductance is a value of when the rotor of the electric motor is magnetically saturated.