To provide an assist device making desired shift range operation not disturbed by influence of electromagnetic wave.

This assist device 1 assists operation force of a shift range operation by detecting operation torque transmitted to an automatic transmission 3 from a shift lever 4a by a magnetic distortion type torque sensor 5, and performing assist drive of a drive motor 6 by converting the same to electrical signal. An electromagnetic wave sensor 12 is provided near the magnetic distortion type torque sensor 5 or a signal transmission route from the magnetic distortion type torque sensor 5. A control part 8 performing a fail safe process stopping assist of operation force of a shift range operation by the drive motor 6 if electromagnetic wave of a fixed value or greater is detected by the electromagnetic wave sensor 12.