A controller for an electric motor comprises: an estimator configured to estimate a position of a rotor by using a d-axis current, a q-axis current, a d-axis voltage command value, and a q-axis voltage command value from which a noise is removed by a filter having a first time constant when a torque command value is equal to or greater than zero, and estimate a rotational speed of the electric motor and the position by using the d-axis current, the q-axis current, the d-axis voltage command value, and the q-axis voltage command value from which a noise is removed by a filter having a second time constant smaller than the first time constant when the torque command value is less than zero.