A synchronous motor driving system including a synchronous motor, a PWM signal controller for conducting pulse width modulation on three-phase voltage command signals by using a carrier signal, and a power converter driven by a gate signal subjected to pulse width modulation, the synchronous motor driving system comprising: command voltage generation means for generating three-phase voltage command signals to apply a high frequency voltage to the synchronous motor; current detection means for detecting a current which flows through the synchronous motor when the high frequency voltage (Vdc) is applied to the synchronous motor; and magnetic pole position estimation means for estimating a magnetic pole position of the synchronous motor on the basis of a result of the current detection, characterized in that the command voltage generation means is adapted to generate the three-phase voltage command signals (Vu*, Vv*, Vw*) by conducting successive switching among a first applied voltage mode for applying a positive-negative alternating high frequency square wave voltage (Vdc) having arbitrary voltage amplitude (Vamp) to an arbitrary phase θ, a second applied voltage mode for applying a high frequency square wave voltage (Vdc) having similar amplitude to a phase shifted in electric angle from the phase θ by 120 degrees, and a third applied voltage mode for applying a high frequency square wave voltage (Vdc) having similar amplitude to a phase shifted in electric angle from the phase θ by 240 degrees, wherein the successive switching is conducted for equalizing voltage applying conditions for detected current values of the three phases, the current detection means is adapted to detect positive side values and negative side values of a pulsating current generated by applying the high frequency voltage in the first, second and third applied voltage modes at a rate of one phase in each of the modes, and obtaining information of a positive side U-phase detected current value (Iu+), a negative side U-phase detected current value (Iu-), a positive side V-phase detected current value (Iv+), a negative side V-phase detected current value (Iv-), a positive side W-phase detected current value (Iw+), and a negative side W-phase detected current value (Iw-), and the magnetic pole position estimation means is adapted to calculate an estimated magnetic pole position of the synchronous motor by using detected current values (Iu+, Iv+, Iw+) of respective phases of the positive side and detected current values (Iu-, Iv-, Iw-) of respective phases of the negative side. command voltage generation means for generating three-phase voltage command signals to apply a high frequency voltage to the synchronous motor; current detection means for detecting a current which flows through the synchronous motor when the high frequency voltage (Vdc) is applied to the synchronous motor; and magnetic pole position estimation means for estimating a magnetic pole position of the synchronous motor on the basis of a result of the current detection, characterized in that the command voltage generation means is adapted to generate the three-phase voltage command signals (Vu*, Vv*, Vw*) by conducting successive switching among a first applied voltage mode for applying a positive-negative alternating high frequency square wave voltage (Vdc) having arbitrary voltage amplitude (Vamp) to an arbitrary phase θ, a second applied voltage mode for applying a high frequency square wave voltage (Vdc) having similar amplitude to a phase shifted in electric angle from the phase θ by 120 degrees, and a third applied voltage mode for applying a high frequency square wave voltage (Vdc) having similar amplitude to a phase shifted in electric angle from the phase θ by 240 degrees, wherein the successive switching is conducted for equalizing voltage applying conditions for detected current values of the three phases, the current detection means is adapted to detect positive side values and negative side values of a pulsating current generated by applying the high frequency voltage in the first, second and third applied voltage modes at a rate of one phase in each of the modes, and obtaining information of a positive side U-phase detected current value (Iu+), a negative side U-phase detected current value (Iu-), a positive side V-phase detected current value (Iv+), a negative side V-phase detected current value (Iv-), a positive side W-phase detected current value (Iw+), and a negative side W-phase detected current value (Iw-), and the magnetic pole position estimation means is adapted to calculate an estimated magnetic pole position of the synchronous motor by using detected current values (Iu+, Iv+, Iw+) of respective phases of the positive side and detected current values (Iu-, Iv-, Iw-) of respective phases of the negative side.