An accelerator opening degree detection device comprising: a handlebar, an accelerator grip which is mounted on one end portion of the handlebar, a rotor mounted on the handlebar around an axis of the handlebar, the rotor being capable of being turned in unison with the accelerator grip as the accelerator grip is turned; a magnet (36a, 36b) mounted on the rotor; a magnetic sensor (30a, 30b) for detecting a magnetic force of the magnet (36a, 36b); a case housing the rotor and the magnetic sensor (30a, 30b), the case being mounted on the handlebar; and a sensor holder housed in the case at a position remote from the accelerator grip with the rotor interposed therebetween, the magnetic sensor (30a, 30b) being installed in the sensor holder; wherein the sensor holder includes an inner projected portion positioned radially inward of the rotor, and projecting toward a position confronting the magnet (36a, 36b); wherein the magnetic sensor (30a, 30b) is disposed in the inner projected portion; and the accelerator opening degree detection device further comprises: a back yoke (38a, 38b) mounted on the rotor radially outward of the magnet (36a, 36b); characterized in that the accelerator opening degree detection device further comprises: a return spring having one end thereof supported in the rotor and another end thereof supported in the sensor holder, the return spring being thereby for normally biasing the rotor to move to a turning start point; wherein the rotor has a groove defined therein, which houses the return spring between the groove and the sensor holder and the return spring has one end that engages and is supported by a hole defined in the rotor and another end that engages and is supported by a hole defined in the sensor holder. a handlebar, an accelerator grip which is mounted on one end portion of the handlebar, a rotor mounted on the handlebar around an axis of the handlebar, the rotor being capable of being turned in unison with the accelerator grip as the accelerator grip is turned; a magnet (36a, 36b) mounted on the rotor; a magnetic sensor (30a, 30b) for detecting a magnetic force of the magnet (36a, 36b); a case housing the rotor and the magnetic sensor (30a, 30b), the case being mounted on the handlebar; and a sensor holder housed in the case at a position remote from the accelerator grip with the rotor interposed therebetween, the magnetic sensor (30a, 30b) being installed in the sensor holder; wherein the sensor holder includes an inner projected portion positioned radially inward of the rotor, and projecting toward a position confronting the magnet (36a, 36b); wherein the magnetic sensor (30a, 30b) is disposed in the inner projected portion; and the accelerator opening degree detection device further comprises: a back yoke (38a, 38b) mounted on the rotor radially outward of the magnet (36a, 36b); characterized in that the accelerator opening degree detection device further comprises: a return spring having one end thereof supported in the rotor and another end thereof supported in the sensor holder, the return spring being thereby for normally biasing the rotor to move to a turning start point; wherein the rotor has a groove defined therein, which houses the return spring between the groove and the sensor holder and the return spring has one end that engages and is supported by a hole defined in the rotor and another end that engages and is supported by a hole defined in the sensor holder. a back yoke (38a, 38b) mounted on the rotor radially outward of the magnet (36a, 36b); characterized in that the accelerator opening degree detection device further comprises: a return spring having one end thereof supported in the rotor and another end thereof supported in the sensor holder, the return spring being thereby for normally biasing the rotor to move to a turning start point; wherein the rotor has a groove defined therein, which houses the return spring between the groove and the sensor holder and the return spring has one end that engages and is supported by a hole defined in the rotor and another end that engages and is supported by a hole defined in the sensor holder.