A motor rotation angle detection device, comprising a resolver which detects a rotation angle of a motor generating an assist torque for steering road wheels. The resolver includes an annular rotor disposed inside an annular stator with an air gap interposed in between an outer circumferential surface of the rotor and an inner circumferential surface of the stator, so that the stator and the rotor are rotatable relative to each other, wherein a rotor supporting member is connected to a rotary shaft of the motor, and the rotor extends out from an axial end portion of the rotor supporting member so as to surround an outer periphery of the rotary shaft. With the such a motor rotation angle detection device, it is possible to stabilize the magnetic permeability of the rotor, and accordingly to increase an accuracy with which the rotation angle of the motor is detected.