An accelerator opening degree detection device is equipped with: a rotor that is attached to the shaft of a handlebar and rotates in conjunction with the operation of an accelerator grip; magnets attached to the rotor; magnetic sensors that detect the magnetic force of the magnets; a case that houses the rotor and the magnetic sensors, and is attached to the handlebar; and a sensor holder to which the magnetic sensors are attached, and which is housed in the case on the opposite side of the rotor with respect to the accelerator grip. The sensor holder has an inner diameter protruding part that protrudes at a location opposing the magnets, this location being on the inside of the rotor in the circumferential direction, and the magnet sensors are arranged on the inner diameter protruding part.