A magnet temperature estimating system for a synchronous electric motor having a permanent magnet, characterized in that the magnet temperature estimating system comprises: a current controller configured to output first voltage command values (vd0*, vq0*) based on current command values (idsf*, iqsf*) and detected fundamental wave currents (idsf, iqsf); a resonant controller configured to generate harmonic command values (vdsc*, vqsc*) based on a differential of harmonic current command values (idsc*, iqsc*) and detected currents (ids, iqs); an adder configured to superimpose the harmonic voltage command values (vdsc*, vqsc*) of a frequency different from a frequency of a fundamental wave driving a synchronous electric motor on the first voltage command values (vd0*, vq0*), and output second voltage command values (vds*, vqs*); a coordinate transformation configured to perform a coordinate transformation on the second voltage command values (vds*, vqs*), and to output three-phase voltage command values (vu*, vv*, vw*); an electric power converter configured to apply a three-phase voltage to the synchronous electric motor on the basis of the three-phase voltage command values (vu*, vv*, vw*), and to drive the synchronous electric motor by a three-phase current (iu, iv, iw) from the electric power converter; a calculator comprised in a magnet temperature estimating unit and configured to calculate an impedance (Zds) of the synchronous electric motor from the superimposed harmonic voltage command values (vdsc*, vqsc*) and currents (idsc*, iqsc*) obtained by the superimposing; and to output a calculated real part (Rd) of the impedance (Zds); and the magnet temperature estimating unit configured to estimate a temperature (Tm) of the permanent magnet on the basis of the calculated real part (Rd) of the impedance (Zds). a current controller configured to output first voltage command values (vd0*, vq0*) based on current command values (idsf*, iqsf*) and detected fundamental wave currents (idsf, iqsf); a resonant controller configured to generate harmonic command values (vdsc*, vqsc*) based on a differential of harmonic current command values (idsc*, iqsc*) and detected currents (ids, iqs); an adder configured to superimpose the harmonic voltage command values (vdsc*, vqsc*) of a frequency different from a frequency of a fundamental wave driving a synchronous electric motor on the first voltage command values (vd0*, vq0*), and output second voltage command values (vds*, vqs*); a coordinate transformation configured to perform a coordinate transformation on the second voltage command values (vds*, vqs*), and to output three-phase voltage command values (vu*, vv*, vw*); an electric power converter configured to apply a three-phase voltage to the synchronous electric motor on the basis of the three-phase voltage command values (vu*, vv*, vw*), and to drive the synchronous electric motor by a three-phase current (iu, iv, iw) from the electric power converter; a calculator comprised in a magnet temperature estimating unit and configured to calculate an impedance (Zds) of the synchronous electric motor from the superimposed harmonic voltage command values (vdsc*, vqsc*) and currents (idsc*, iqsc*) obtained by the superimposing; and to output a calculated real part (Rd) of the impedance (Zds); and the magnet temperature estimating unit configured to estimate a temperature (Tm) of the permanent magnet on the basis of the calculated real part (Rd) of the impedance (Zds).