Provided are a power converter for a vehicle generator-motor and a method for controlling a vehicle generator-motor, which is used for a field winding type vehicle generator-motor configured to perform control in accordance with an external operating command in which a power converter is connected to a rotating electrical machine including a field winding and an armature winding. When the generator-motor shifts from a driving state to another operating mode, a method of stopping a driving mode is switched in accordance with an operating mode to be shifted, to thereby shift the operating mode promptly without generating an unintended power generation, an excess power generation, and an excess torque fluctuation.