Throttle opening detection device

To miniaturize a throttle opening detection device in the direction along the radial direction of a rotor. [Means for Resolution] A throttle opening detection device 10 includes a rotor 16 which is rotated in an interlocking manner by manipulation of an acceleration grip 14. Magnets 28a, 28c are arranged in the vicinity of a peripheral portion of one bottom surface of the rotor 16 along the circumferential direction of the rotor 16. Magnets 28b, 28d are laminated to the magnets 28a, 28c respectively. A predetermined clearance is preferably formed between the magnets 28a, 28b and the magnets 28c, 28d. A Hall element 30 (magnetic sensor) for detecting a magnetic force generated by these magnets 28a to 28d is arranged to face one bottom surface of the rotor 16, preferably, the clearance in an opposed manner.